Fixes #3137.

Changes:

1. Unlock the mutex of m_RcvBufLock, which should guard the ACK sequence recorded by another thread. The lock is not necessary at the moment when the prepared packet is about to be sent.
2. As the nonblocking mode was set using either O_NONBLOCK flag (by fcntl) or short timeout on the socket, depending on the system, for consistency the non-blocking mode should be set in both directions. Therefore added setting also SO_SNDTIMEO flag with the same value.