Preserve some logcontexts

Add None checks to backfill

Pass through user-supplied content in /join/$room_id

It was always intended to allow custom keys on the join event, but this has
at some point been lost. Restore it.

If the user specifies keys like "avatar_url" then they will be clobbered.

Delete old received_transactions rows

Add the Notifications API

Add usage stats to prometheus monitoring

Add more Measure blocks

Matthew/workerdoc

Add concept of cache contexts