librados: watch_flush() on shutdown

Users can easily forget this. It makes shutdown potentially block, but if
they have racing callbacks they get what they ask for.

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 30678f6daf)

src/librados/RadosClient.cc

@@ -282,6 +282,9 @@ int librados::RadosClient::connect()
 
 void librados::RadosClient::shutdown()
 {
+  // make sure watch callbacks are flushed
+  watch_flush();
+
   lock.Lock();
   if (state == DISCONNECTED) {
     lock.Unlock();