Implemented #648.

[redboltz]

Added PINGRESP timeout.
If the timeout value is set, when PINGREQ is sent but PINGRESP is not
received until the timeout, then force_disconnect() is called from the
client side internally.

[redboltz]

steady_timer is an alias of basic_waitable_timer< chrono::steady_clock >.

Its constructor requires io_context& until 1.69.0
https://www.boost.org/doc/libs/1_69_0/doc/html/boost_asio/reference/basic_waitable_timer/basic_waitable_timer.html
Since 1.70.0, executor const& version is introduced.
https://www.boost.org/doc/libs/1_70_0/doc/html/boost_asio/reference/basic_waitable_timer/basic_waitable_timer.html

When creating a server side endpoint, socket is passed. I can get executor from the socket using https://www.boost.org/doc/libs/1_73_0/doc/html/boost_asio/reference/basic_stream_socket/get_executor.html .
But I can't get io_context.
In order to get io_context, I need to pass io_context.

If the minimum boost version support updates to 1.70.0, I can use executor based approach.
If it is too new, I need to pass io_context.

[codecov]

Codecov Report

Merging #649 into master will decrease coverage by 0.59%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #649      +/-   ##
==========================================
- Coverage   85.12%   84.53%   -0.60%     
==========================================
  Files          40       41       +1     
  Lines        5682     6544     +862     
==========================================
+ Hits         4837     5532     +695     
- Misses        845     1012     +167     

[jonesmz]

I have been working on moving away from boost 1.67.
I am now on boost 1.68.
I want to use boost 1.70 within 60 days.

I am OK with change of boost requirement to 1.70 today. Would prefer we wait until I have updated my version of boost, but do not know timeline, so I don't want to hold up the mqtt_cpp project.

[redboltz]

@jonesmz , thnaks.

I noticed that I can pass a reference io_context without big change. So this PR can work with boost 1.67.0.