CUDTUnited::m_GlobControlLock takes too much responsibility

[maxsharabayko]

It looks like the CUDTUnited::m_GlobControlLock has to protect:

• CUDTUnited::m_Sockets,
• CUDTUnited::m_Groups,
• CUDTUnited::m_PeerRec,
• CUDTUnited::m_ClosedSockets,
• CUDTUnited::m_ClosedGroups.

It also occasionally protects

• CUDTUnited::m_mMultiplexer, although there is a dedicated m_mMultiplexerLock. See CUDTUnited::updateListenerMux(..).
• WTF CUDTUnited::bind() locks CUDTSocket::m_ControlLock?

TODO

• Define the scope of the CUDTUnited::m_GlobControlLock mutex.
• Clean up the use of the CUDTUnited::m_GlobControlLock mutex.
• Update the low level info document.

Epic #1610.

[gou4shi1]

related issue #2252