* Use one goroutine per user to avoid locking up the entire server for
one bad push gateway.
* Split pushing by format.
* Send one device per push. Sytest does not support coalescing
multiple devices into one push. Matches Synapse. Either we change
Sytest, or remove the group-by-url-and-format logic.
* Write OutputNotificationData from push server. Sync API is already
the consumer.
