Amalgamate _filter_events_for_client

Encode the filter JSON as UTF-8 before storing in the database.

Because we are using a binary column type to store the filter JSON.

Update the v2 sync API to work as specified in the current spec.

Remove unused room_id arg

Bounce all deferreds through the reactor to make debugging easier.

If all deferreds wait a reactor tick before resolving then there is
always a chance to add an errback to the deferred so that stacktraces
get reported, rather than being discarded.

Update the v2 filters to support filtering presence.

Split out the schema preparation and update logic into its own module