Fixes up #17333, where we failed to actually send less data (the
`DISTINCT` didn't work due to `stream_id` being different).

We fix this by making it so that every device list outbound poke for a
given user ID has the same stream ID. We can't change the query to only
return e.g. max stream ID as the receivers look up the destinations to
send to by doing `SELECT WHERE stream_id = ?`