#8567 started a span for every background process. This is good as it means all Synapse code that gets run should be in a span (unless in the sentinel logging context), but it means we generate about 15x the number of spans as we did previously.
This PR attempts to reduce that number by a) not starting one for send commands to Redis, and b) deferring starting background processes until after we're sure they're necessary.
I don't really know how much this will help.
* 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.
This is for two reasons:
1. Suppresses duplicates correctly, as the notifier doesn't do any
duplicate suppression.
2. Makes it easier to connect the AppserviceHandler to the replication
stream.
Access it directly from the homeserver itself. It already wasn't
inheriting from BaseHandler storing it on the Handlers object was
already somewhat dubious.
Bug introduced in 92b20713d7
which reversed the comparison when checking if a user existed
in the users table. Added UTs to prevent this happening again.
