Fix not sending events over federation when using sharded event persisters (#8536)

* Fix outbound federaion with multiple event persisters.

We incorrectly notified federation senders that the minimum persisted
stream position had advanced when we got an `RDATA` from an event
persister.

Notifying of federation senders already correctly happens in the
notifier, so we just delete the offending line.

* Change some interfaces to use RoomStreamToken.

By enforcing use of `RoomStreamTokens` we make it less likely that
people pass in random ints that they got from somewhere random.