TcpSession::Send() and TcpSession::WriteReady() can be called from
concurrent threads and hence just having an atomic bool to update
the send status is not enough to guarantee that the correct send
status is finally set. A recent change in SandeshWriter removed
the mutex acquired before called TcpSession::Send() and updating the
send status and used to update the send status in the
TcpSession::WriteReady() callback. This change reverts back to
using the mutex in SandeshWriter.
Closes-Bug: #1388221

Change-Id: Ibb560021d6a0d8963db6284f1d722a974cb45be2