[core] fix CRcvBufferNew::m_iStartSeqNo was not sync in group

[gou4shi1]

Use m_RecvLock instead of m_RcvBufferLock since the above m_pRcvBuffer->applyGroupTime() is protected by m_RecvLock, maybe we should change to m_RcvBufferLock consistently?

[maxsharabayko]

The m_RecvLock is intended to synchronise srt_recvmsg2(..) API calls between each other if called from multiple threads, and with the TSBPD thread, in particular, to exchange read-readiness state updates in the blocking mode. The mutex is indeed used in too many places to protect access to the receiver buffer, where m_RcvBufferLock must be used for that purpose instead.

[maxsharabayko]

Created a separate issue #2304 to address the use of the m_RecvLock mutex.

[gou4shi1]

Thanks!