`partial_state_rooms` has an FK on `events` pointing to the join event we
get from `/send_join`, so we must delete from that table before deleting
from `events`.

**NB:** It would be nice to cancel any resync processes for the room
being purged. We do not do this at present. To do so reliably we'd need
an internal HTTP "replication" endpoint, because the worker doing the
resync process may be different to that handling the purge request.

The first time the resync process tries to write data after the deletion
it will fail because we have deleted necessary data e.g. auth
events. AFAICS it will not retry the resync, so the only downside to
not cancelling the resync is a scary-looking traceback.

(This is presumably extremely race-sensitive.)

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>