queue: notification queue can become stuck if some handlers are not found

[pierre]

When a node claims some entries, but cannot find the NotificationQueueHandler for them, the entries are left claimed in the database:

https://github.com/killbill/killbill-commons/blob/master/queue/src/main/java/com/ning/billing/notificationq/NotificationQueueDispatcher.java#L184

This will result in another node eventually stealing them from the first node. If the handler is never present, the notification queue can become stuck (by always claiming these entries which cannot be processed).

[sbrossie]

@pierre Should we close this?

[pierre]

I think this is still an issue today.