clarify that `purge_history` will not purge state

clean formatting

WIP, need to add most recent PRs

Register membership/state servlets in event_creator

Implement RoomMemberWorkerHandler

Only update remote profile cache on master

Split up ProfileStore

_remote_join and co take a requester

Add is_blocked to worker store

Fixes a regression that had crept in where the caching layer upholds requests for loading state which is filtered by type (but not by state_key), but the DB layer itself would interpret a missing state_key as a request to filter by null state_key rather than returning all state_keys.

Split RoomMemberHandler into base and master class

I added yields when calling user_left_room, but they shouldn't matter on
the master process as they always return None anyway.

The intention here is to split the class into the bits that can be done
on workers and the bits that have to be done on the master.

In future there will also be a class that can be run on the worker,
which will delegate work to the master when necessary.

Remove ReplicationLayer and user Client/Server directly

Factor out _remote_reject_invite in RoomMember

Don't build handlers on workers unnecessarily