msgr: restart_queue when replacing existing pipe and taking over the queue

The queue may have been previously stopped (by discard_queue()), and needs
to be restarted.

Fixes consistent failures from the mon_recovery.py integration tests.

Signed-off-by: Sage Weil <sage@inktank.com>

src/msg/SimpleMessenger.cc

@@ -860,6 +860,7 @@ int SimpleMessenger::Pipe::accept()
     in_q = existing->in_q;
     in_q->lock.Lock();
     in_q->pipe = this;
+    in_q->restart_queue();
     in_q->lock.Unlock();
     existing->in_q = new IncomingQueue(msgr->cct, existing);