[Bug fix] Release global variables

[poohRui]

解决老版本CtrlServer在python端不会挂的问题，部分Global对象没有被释放。

[jackalcooper]

合并了这个改动之后，经常出现奇怪的错误。可能是因为调用Global<EnvGlobalObjectsScope>::Delete()之后有其他地方调用了用到 env 中包含的对象/内存。在没有完全梳理好生命周期之前（可能和 python session 等代码移到 cpp 相关），是不是可以考虑先不调用这个 delete，因为这样让系统可靠性下降。
出现的错误有：

free(): invalid next size (fast)
Fatal Python error: Aborted

或者

#13 0x00007f99f1bef41b in oneflow::MemoryAllocatorImpl::Deallocate(void*, oneflow::MemoryCase) ()
   from /python_scripts/oneflow/_oneflow_internal.cpython-36m-x86_64-linux-gnu.so
(gdb) 
#14 0x00007f99f1befdc2 in oneflow::MemoryAllocator::Deallocate(char*, oneflow::MemoryCase) ()
   from /python_scripts/oneflow/_oneflow_internal.cpython-36m-x86_64-linux-gnu.so
(gdb) 
#15 0x00007f99f1bf03e3 in std::_Function_handler<void (), std::_Bind<void (oneflow::MemoryAllocator::*(oneflow::MemoryAllocator*, char*, oneflow::MemoryCase))(char*, oneflow::MemoryCase)> >::_M_invoke(std::_Any_data const&) () from /python_scripts/oneflow/_oneflow_internal.cpython-36m-x86_64-linux-gnu.so
(gdb) 
#16 0x00007f99f1bef608 in oneflow::MemoryAllocator::~MemoryAllocator() () from /python_scripts/oneflow/_oneflow_internal.cpython-36m-x86_64-linux-gnu.so
(gdb) 
#17 0x00007f99f1a84c6f in oneflow::Runtime::DeleteAllGlobal() () from /python_scripts/oneflow/_oneflow_internal.cpython-36m-x86_64-linux-gnu.so
(gdb) 
#18 0x00007f99f1a854be in oneflow::Runtime::~Runtime() () from /python_scripts/oneflow/_oneflow_internal.cpython-36m-x86_64-linux-gnu.so
(gdb) quit