mon: shutdown finisher in a more graceful way

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>

src/mon/MonClient.cc

@@ -427,6 +427,7 @@ void MonClient::shutdown()
   monc_lock.Unlock();
 
   if (initialized) {
+    finisher.wait_for_empty();
     finisher.stop();
   }
   monc_lock.Lock();