[core] Added busy counter for sockets and various fixes for data race problems

[ethouris]

Here is introduced the SocketKeeper class, similar to GroupKeeper, together with the busy counter. The goal of this solution is to keep the socket alive until the busy counter reaches 0. This allows various facilities that are still running after the socket is requested to be closed to finish their job without having the socket been physically deleted in the meantime (in the code there were many places with a high risk of this to happen).

The overall procedure is that the socket when closed is being moved from m_Sockets to m_ClosedSockets and after this happens the socket is no longer dispatchable through CUDTUnited::locateSocket. After this happens, the cyclic call to GC may potentially delete the socket. But there can be still some activities running in other threads that might use this socket even when simultaneously moved to m_ClosedSockets. This temporary container is intended to be used for that exactly purpose, to keep a socket that should no longer be visible until all threads finish using it. Various checks are being done in CUDT::checkBrokenSockets to prevent deletion of sockets that are still in use by other threads, but new features in SRT have added extra situations when this may happen and it has been observed in the ThreadSanitizer that - although not fatal - an attempt to delete a socket that is simultaneously used to send a packet was detected.

To prevent this, in various longer procedures that might be potentially caught in the middle of deletion of a socket there is applied a SocketKeeper. This does RAII operation on the m_iBusy counter so that it is set back to the previous value only when this procedure is finished. The loop over the m_ClosedSockets that selects the sockets to delete will check this and if this busy counter is not zero, deletion of a socket will miss this cycle (will have to wait for the next GC cycle to try again).

A small controversy might exist about the free acquire/release functions and that's why the use of them should be exclusive to SocketKeeper so that the RAII mechanism can ensure that a temporary busy counter will always be withdrawn when no longer needed, and no such situation is permanent.

[codecov]

Codecov Report

Attention: Patch coverage is 64.81481% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 64.67%. Comparing base (4f925fb) to head (212750d).
Report is 9 commits behind head on master.

❗ Current head 212750d differs from pull request most recent head f56dba5. Consider uploading reports for the commit f56dba5 to get more accurate results

Files	Patch %	Lines
srtcore/api.cpp	52.00%	12 Missing ⚠️
srtcore/queue.cpp	50.00%	6 Missing ⚠️
srtcore/core.cpp	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2893      +/-   ##
==========================================
- Coverage   64.88%   64.67%   -0.21%     
==========================================
  Files         101      101              
  Lines       17634    17565      -69     
==========================================
- Hits        11441    11360      -81     
- Misses       6193     6205      +12     

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