more metrics for the federation and appservice senders

Support for room versioning

Reserved users for MAU limits

API for monthly_active_users table

Fix occasional glitches in the synapse_event_persisted_position metric

Every so often this metric glitched to a negative number. I'm assuming it was
due to backfilled events.

Reject make_join requests from servers which do not support the room version.

Also include the room version in the response.

This is the first tranche of support for room versioning. It includes:
 * setting the default room version in the config file
 * new room_version param on the createRoom API
 * storing the version of newly-created rooms in the m.room.create event
 * fishing the version of existing rooms out of the m.room.create event

... to save me reverse-engineering this stuff again.