It's closer to a presence query than an update, and more importantly
this moves this out of Tornado -- previously Tornado was spending at
least 3ms per recipient on messages sent to the MIT realm fetching all
this data to return back to users. This should save around 100ms per
message sent to a popular stream the MIT realm -- but more
importantly, each such event is 100ms during which Tornado is not
processing other messages.
(imported from commit 134169f0fdcd9f6640fda957edc4a28b07783d8e)