We extract get_bulk_stream_subscriber_info() from this
function to remove some of the complexity. Also, in that
new function we avoid a hop to the database by querying
on stream ids instead of recipient ids. The query that
gets changed here does require a join to the recipient
table (to get the stream id), so it's a little bit of a
tradeoff.
The first method we extract to this library is
get_active_subscriptions_for_stream_id().
We also move num_subscribers_for_stream_id() to here, which
is slightly annoying (having the method on Stream was nice)
but avoids some circular dependency issues.