Send device messages for the same user in same API call.
Currently we split the device messages up to limit the number per call,
but that can end up splitting messages to a given users device over
separate API calls. This is fine, but means that the server can't e.g.
bundle them into a single EDU for remote users or sanity check that the
client is sending to the right set of devices (i.e. its device list
cache isn't wrong).
