This new module tracks the recent topic names for any given
stream.
The code was pulled over almost verbatim from stream_data.js,
with minor renames to the function names.
We introduced a minor one-line function called stream_has_topics.
We now have all of our callers into recent_topics code just
receive a list of topic names from get_recent_topic_names().
This is more encapsulated than handing off tiny little
structures to the three callers, two of whom immediately
mapped the objects to names, and one of whom needlessly
used the now defunct name canon_subject field.
The consolidation here removes some "subject" references, and
now all lookup are by stream id, not stream name.
The diff here is a bit daunting, but it's mostly simplification
of tests and calling code. Two of the callers now need to look
up stream ids, but they are otherwise streamlined.
The main change here is to stream_data.js, and we replace the
`canon_subject` and `subject` fields with `name`.
We now use "map" to have our inner generator of topics be
mapped to objects with both the stream and topic. Thanks to
Mahim Goyal for helping with this design.
Steve Howell
Joshua Pan