[core] Disconnect dead sockets from the multiplexer to free unused resources

[ethouris]

Fixes: #1822

1. Refax.
2. Added removal from multiplexer at the moment when the socket is moved to m_ClosedSockets. As the remaining activities are still pending, the socket should stop using the multiplexer, and if it was the last user of the multiplexer, the multiplexer will be deleted and will free the system UDP socket.

CONTROVERSY: The multiplexer ships also the m_pRcvQueue and m_pSndQueue objects that are pointer-copied into the CUDT structure. This might be then used by other threads, and the only lock that was observed to be guarding them is the m_ConnectionLock, which's range seems to be limited, and also this locks orders before m_GlobControlLock. The only collision observed so far in manual tests was that when closing the queue is used if there are set connection flags, so therefore additionally these flags are cleared. Not sure if this is enough to prevent the methods of CUDT class called from other threads to refer to m_pSndQueue or m_pRcvQueue. What is known is that likely at the time of moving the socket to ClosedSockets some of the methods executed in other threads might be in progress.

[ethouris]

All current attempts to fix this have failed. The current solution makes a hangup due to not cleaned up all socket resources.