BUG-5280: Fix deadlock with TransmitQueue

BUG-5280: Fix deadlock with TransmitQueue

TransmitQueue should not be dispatching responses while
the connection lock is being held, as a that may expose
clients to AB/BA deadlocks if the callback tries to acquire
a lock which is being held by another thread attempting
to touch the same connection (i.e. transmit).

Fix this by TransmitQueue returning the entry to be
completed and AbstractClientConnection checking its
presence and performing actual completion after it has
dropped the connection lock.

Change-Id: Ibbacb641a297bfe7d4790af8401b036285c26593
Signed-off-by: Robert Varga <rovarga@cisco.com>