Avoid long locks in process missedmessage_emails queue.

(imported from commit aed4a89244dce19085ed2ad89a7539ab297d8bc9)

zerver/worker/queue_processors.py

@@ -160,6 +160,7 @@ class MissedMessageWorker(QueueProcessingWorker):
                 by_recipient[event['user_profile_id']].append(event)
 
             for user_profile_id, events in by_recipient.items():
+                with commit_on_success():
                     handle_missedmessage_emails(user_profile_id, events)
 
             # Aggregate all messages received every 2 minutes to let someone finish sending a batch