msg/DispatchQueue: clear queue after wait()

We can have a sequence like

 - put several things in mqueue
 - call stop(), which makes thread exit without processing all items
 - wait() waits for thread to exit

...and have items left in the queue.  Throw them out on wait() (which is
called post-shutdown()).

Fixes: http://tracker.ceph.com/issues/18351
Signed-off-by: Sage Weil <sage@redhat.com>

src/msg/DispatchQueue.cc

@@ -237,6 +237,13 @@ void DispatchQueue::wait()
 {
   local_delivery_thread.join();
   dispatch_thread.join();
+
+  // ok, clear it out
+  lock.Lock();
+  while (!mqueue.empty()) {
+    QueueItem i = mqueue.dequeue();
+  }
+  lock.Unlock();
 }
 
 void DispatchQueue::discard_local()