Improve Socket ID generation to better occupy the valid range

[maxsharabayko]

CUDTUnited::generateSocketID(..) uses the same m_SocketIDGenerator variable both for a single socket and a group socket ID generation.
It means that generating a single socket value e.g. 100, decrements m_SocketIDGenerator by one. The resulting Socket ID value is 100.
Next generating a group socket ID again decrements the value of m_SocketIDGenerator by one. The resulting Group Socket ID value is 99 | GROUP_MASK.
Generating next Socket ID produces the value 98, skipping value 98.
When m_SocketIDGenerator reaches its initial value of 100 (saved in m_SocketIDGenerator_init, no more sockets are generated with the default procedure.
However, some SocketIDs are still available.

So we get into checking iterating over occupied socket IDs in the loop under if (m_SocketIDGenerator_init == 0). From now on any new socket ID generation goes though this loop.

Using a separate m_GroupSocketIDGenerator variable could save some time before going into this procedure.

Also a check whether all Socket IDs are occupied: m_Sockets.size() and m_Groups.size() could be added.

SRT Version: v1.4.2,