* 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.
This manipulates account data over User API, and fires sync messages
for changes. Those sync messages should, according to an existing TODO
in clientapi, be moved to userapi.
Forks clientapi/producers/syncapi.go to pushserver/ for later extension.
