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
hmm, yes and no. it's still per-Source, so if it's seen an Instagram url before, that won't prevent it from finding a new Twitter url in the future (that's the common case for me, where OwnYourGram automatically PESOS to me, and then I manually syndicate out later)

but it would prevent it from finding updated/additional links for the same source in the future, which I can definitely imagine happening (esp. with the multiple urls for FB photos) but should be much less common.