brand the email from header

Change CacheMetrics to be quicker

Poke notifier on next reactor tick

Make get_joined_hosts_for_room use get_users_in_room

Add methods to events, account data and receipt slaves

Load push rules in storage layer so that they get cached

We change it so that each cache has an individual CacheMetric, instead
of having one global CacheMetric. This means that when a cache tries to
increment a counter it does not need to go through so many indirections.

Add a slaved store for presence

Adds the methods needed by /sync to the slaved events,
account data and receipt stores.

Add slaved stores for filters, tokens, and push rules

Use state to calculate get_users_in_room

Add get_users_with_read_receipts_in_room cache

Email unsubscribe links that don't require logging in

Fix error in email notification string formatting

and authed by macaroons that let you delete pushers and nothing else

Rather than storing them as UserID objects.