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

- 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.
posse-post-discovery: refactoring and cleanup
posse-post-discovery: append to unsent list

was incorrectly replacing the unsent list, but tasks might not
be complete yet.
posse-post-discovery: SyndicatedPosts unit tests
posse-post-discovery: ensure last_syndication_url is set
Merge pull request #192 from kylewm/ppd-update-syndication-urls

posse-post-discovery: periodically refetch h-feed