Continues from uhoreg's branch
This just fixed the errcode on /room_keys/version if no backup and
updates the schema delta to be on the latest so it gets run
Create the url_cache index on local_media_repository as a background update, so
that we can detect whether we are on sqlite or not and create a partial or
complete index accordingly.
To avoid running the cleanup job before we have built the index, add a bailout
which will defer the cleanup if the bg updates are still running.
Fixes https://github.com/matrix-org/synapse/issues/2572.
Prevent group API access to non-members for private groups
Also make all the group code paths consistent with `requester_user_id` always being the User ID of the requesting user.
