librbd: TaskFinisher should finish all queued tasks

The destructor wasn't waiting for all Finisher tasks
to complete before stopping the thread.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 8e20240)

src/librbd/TaskFinisher.h

@@ -35,6 +35,7 @@ class TaskFinisher {
       delete m_safe_timer;
     }
 
+    m_finisher->wait_for_empty();
     m_finisher->stop();
     delete m_finisher;
   }