Implement notify/listen for queue pending tasks

[lotas]

After migrating from single messages table, queue service can now use database more efficiently, but there's still room for improvement.

Queue service still uses polling to fetch new tasks for queues, and this can be improved with using notify/listen in postgres

This is the last item remaining from #2940 (comment) migration

[djmitche]

I suspect this will remove a lot of load from the DB servers! 🚀