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
the advantage of storing `Response.url` would be that I could query only the small set of Responses that are newly discovered. as long as url was follow_redirectsed and canonicalized before it was stored, that would be really nice.

couple of disadvantages:
1. I'd expect pulling URL from activity_json to work for older Responses (i.e. from before this goes into production). am I missing something that will prevent that?
2. Making the URLs canonical before storing makes it a little harder to change the canonicalization method in the future.