Fix that on ubuntu 20.04, the current version of the code does not co…

…mpile as a python package

Steps to reproduce:

```
  # install required packages
  apt-get install build-essential libncurses5-dev libpcap-dev pybind11-dev

  # create working directory
  mkdir nethogs-py
  cd nethogs-py

  # create virtualenv
  virtualenv -p python3 venv

  # activate virtualenv
  source venv/bin/activate

  # create requirements.txt file for pip
  echo "git+https://github.com/raboof/nethogs.git" > requirements.txt

  # build nethogs using pip
  pip install -r requirements.txt
```

The last step gives the following compile error on my Ubuntu 20.04
machine:

```
Processing ./nethogs
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
    Preparing wheel metadata ... done
Building wheels for collected packages: nethogs
  Building wheel for nethogs (PEP 517) ... error
  ERROR: Command errored out with exit status 1:
   command: /home/lysgaard/work/nethogs-py/venv/bin/python /tmp/tmpbzq5d0ej build_wheel /tmp/tmpxbnq_o9a
       cwd: /tmp/pip-req-build-uxzyh5t6
  Complete output (50 lines):
  running bdist_wheel
  running build
  running build_ext
  x86_64-linux-gnu-gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -fPIC -I/home/lysgaard/work/nethogs-py/venv/include -I/usr/include/python3.8 -c flagcheck.cpp -o flagcheck.o -std=c++17
  building 'nethogs' extension
  creating build
  creating build/temp.linux-x86_64-cpython-38
  creating build/temp.linux-x86_64-cpython-38/python
  creating build/temp.linux-x86_64-cpython-38/src
  x86_64-linux-gnu-gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -fPIC -Isrc/ -I/tmp/pip-build-env-kvg6xf6h/overlay/lib/python3.8/site-packages/pybind11/include -I/home/lysgaard/work/nethogs-py/venv/include -I/usr/include/python3.8 -c python/bindings.cpp -o build/temp.linux-x86_64-cpython-38/python/bindings.o -std=c++17 -fvisibility=hidden -g0 -Wall -Wextra -Wno-missing-field-initializers --std=c++0x -O3 -fPIC -DVERSION=\"0.8.7\"
  x86_64-linux-gnu-gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector-strong -Wformat -Werror=format-security -g -fwrapv -O2 -fPIC -Isrc/ -I/tmp/pip-build-env-kvg6xf6h/overlay/lib/python3.8/site-packages/pybind11/include -I/home/lysgaard/work/nethogs-py/venv/include -I/usr/include/python3.8 -c src/connection.cpp -o build/temp.linux-x86_64-cpython-38/src/connection.o -std=c++17 -fvisibility=hidden -g0 -Wall -Wextra -Wno-missing-field-initializers --std=c++0x -O3 -fPIC -DVERSION=\"0.8.7\"
  src/connection.cpp: In function ‘Connection* findConnectionWithMatchingSource(Packet*, short int)’:
  src/connection.cpp:162:37: error: no matching function for call to ‘std::multiset<Connection*, ConnectionComparator>::lower_bound(Packet*)’
    162 |   auto it = connList->lower_bound(&p);
        |                                     ^
  In file included from /usr/include/c++/9/set:62,
                   from src/process.h:29,
                   from src/connection.cpp:34:
  /usr/include/c++/9/bits/stl_multiset.h:810:7: note: candidate: ‘std::multiset<_Key, _Compare, _Alloc>::iterator std::multiset<_Key, _Compare, _Alloc>::lower_bound(const key_type&) [with _Key = Connection*; _Compare = ConnectionComparator; _Alloc = std::allocator<Connection*>; std::multiset<_Key, _Compare, _Alloc>::iterator = std::_Rb_tree_const_iterator<Connection*>; std::multiset<_Key, _Compare, _Alloc>::key_type = Connection*]’
    810 |       lower_bound(const key_type& __x)
        |       ^~~~~~~~~~~
  /usr/include/c++/9/bits/stl_multiset.h:810:35: note:   no known conversion for argument 1 from ‘Packet*’ to ‘Connection* const&’
    810 |       lower_bound(const key_type& __x)
        |                   ~~~~~~~~~~~~~~~~^~~
  /usr/include/c++/9/bits/stl_multiset.h:814:7: note: candidate: ‘std::multiset<_Key, _Compare, _Alloc>::const_iterator std::multiset<_Key, _Compare, _Alloc>::lower_bound(const key_type&) const [with _Key = Connection*; _Compare = ConnectionComparator; _Alloc = std::allocator<Connection*>; std::multiset<_Key, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<Connection*>; std::multiset<_Key, _Compare, _Alloc>::key_type = Connection*]’
    814 |       lower_bound(const key_type& __x) const
        |       ^~~~~~~~~~~
  /usr/include/c++/9/bits/stl_multiset.h:814:35: note:   no known conversion for argument 1 from ‘Packet*’ to ‘Connection* const&’
    814 |       lower_bound(const key_type& __x) const
        |                   ~~~~~~~~~~~~~~~~^~~
  src/connection.cpp: In function ‘Connection* findConnectionWithMatchingRefpacketOrSource(Packet*, short int)’:
  src/connection.cpp:187:41: error: no matching function for call to ‘std::multiset<Connection*, ConnectionComparator>::lower_bound(Packet*&)’
    187 |   auto it = connList->lower_bound(packet);
        |                                         ^
  In file included from /usr/include/c++/9/set:62,
                   from src/process.h:29,
                   from src/connection.cpp:34:
  /usr/include/c++/9/bits/stl_multiset.h:810:7: note: candidate: ‘std::multiset<_Key, _Compare, _Alloc>::iterator std::multiset<_Key, _Compare, _Alloc>::lower_bound(const key_type&) [with _Key = Connection*; _Compare = ConnectionComparator; _Alloc = std::allocator<Connection*>; std::multiset<_Key, _Compare, _Alloc>::iterator = std::_Rb_tree_const_iterator<Connection*>; std::multiset<_Key, _Compare, _Alloc>::key_type = Connection*]’
    810 |       lower_bound(const key_type& __x)
        |       ^~~~~~~~~~~
  /usr/include/c++/9/bits/stl_multiset.h:810:35: note:   no known conversion for argument 1 from ‘Packet*’ to ‘Connection* const&’
    810 |       lower_bound(const key_type& __x)
        |                   ~~~~~~~~~~~~~~~~^~~
  /usr/include/c++/9/bits/stl_multiset.h:814:7: note: candidate: ‘std::multiset<_Key, _Compare, _Alloc>::const_iterator std::multiset<_Key, _Compare, _Alloc>::lower_bound(const key_type&) const [with _Key = Connection*; _Compare = ConnectionComparator; _Alloc = std::allocator<Connection*>; std::multiset<_Key, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<Connection*>; std::multiset<_Key, _Compare, _Alloc>::key_type = Connection*]’
    814 |       lower_bound(const key_type& __x) const
        |       ^~~~~~~~~~~
  /usr/include/c++/9/bits/stl_multiset.h:814:35: note:   no known conversion for argument 1 from ‘Packet*’ to ‘Connection* const&’
    814 |       lower_bound(const key_type& __x) const
        |                   ~~~~~~~~~~~~~~~~^~~
  error: command '/usr/bin/x86_64-linux-gnu-gcc' failed with exit code 1
  ----------------------------------------
  ERROR: Failed building wheel for nethogs
Failed to build nethogs
ERROR: Could not build wheels for nethogs which use PEP 517 and cannot be installed directly

```

I have tracked this down to that the `setup.py` file uses the wrong
C++-standard-flag.

This is corrected in this commit.

I have also reduced the optimization level from 3 to 2.
The rationale is that optimization level 3 contains
optimazitions that potentially change the behaviour of code, eg.
floating point behaviour. General convention is that optimization level
2 is more than sufficient unless you are working with extremely tight
budget, eg linear algebra kernels or similar. If optimization level 3
is still wanted, I can revert this.

setup.py

@@ -26,8 +26,8 @@
     "-Wall",
     "-Wextra",
     "-Wno-missing-field-initializers",
-    "--std=c++0x",
-    "-O3",
+    "--std=c++17",
+    "-O2",
     "-fPIC",
     '-DVERSION="{}"'.format(__version__)
 ]