prune_old_outbound_device_pokes is burning CPU on the master

[richvdh]

Every hour, it runs for 20 minutes. It's running in a database thread but I suspect it's holding the GIL for a lot of that time,

It's table-scanning the whole table, and then doing a query on every single (user_id,destination) in the table (over a million of them). The code doesn't seem to realise that users might have more than one device. We either need to optimise the query, or move the pruned remnants out to a separate table so that we aren't redoing the same work the whole time.

Related: #4115

[richvdh]

I hope this is fixed by #7159