Observer and locks

`create_observer` takes a deferred and create a new deferred that
*observers* the original deferred. Any callbacks added to the observing
deferred will *not* affect the origin deferred.

Shuffle operations so that locking upsert happens last in the txn. This ensures the lock is held for the least amount of time possible.

Make one-to-one rule an underride otherwise bings don't work in one-to-one wrooms. Likewise a couple of other rules.

Remove users from the remote_offline_serials list (and clean up empty elements) when they go online again

Store a list of the presence serial number at which remote users went offline, so that when we delete them from the cachemap, we can still synthesize OFFLINE events for them (SYN-261)

Sanitise a user's powerlevel to an int() before numerical comparison, because otherwise Python is "helpful" with it (SYN-351)

Unify http wrappers

SYN-350: Don't ratelimit the events generated during create room