posse-post-discovery: periodically refetch h-feed

- look for updated rel=syndication links and re-propagate
  Responses that did not have the syndication url the first
  time it was propagated.

- narrowing the scope of the posse-post-discovery transaction.
  the new refetch behavior made this much more imperative.

- added 3 unit tests, one to original_post_discovery_test
  that exercises refetch(), and 2 to tasks_test to test both
  the situation where we should refetch and the situation where
  we shouldn't.

this is intended to address issues #171 and #131
if i'm following this logic right, we'll handle the case where we see no syndication urls on first fetch, but if we find one and then later the author adds a second, it won't handle that, right? assuming so, we probably want to think about handling that too. that can be a later PR though.