Database performance improvements.

SYN-208/SYN-228: Add runtime checks on startup to enforce that JPEG/PNG support is included when installing pillow.

Add RoomMemberStore.get_users_in_room, so that we can get the list of joined users without having to retrieve the full events

Look for name, topic in the event content rather than the event itself when persisting room name and topic events

Supply auth_chain along with current state in '/state/', fetch auth events from a remote server if we are missing some of them

Replace distributor deferred list, with a simple for loop until I understand why the former breaks and the latter doesn't