posse-post-discovery: reduce number of NDB operations

- do one query for all permalinks on a page, rather than querying
  for each permalink individually
- when refetching blanks, don't delete blanks unless we actually
  find something to replace them with (moved the deletion code
  to SyndicatedPost.get_or_insert... so that it's all part of
  the same transaction now)
one catch with the IN operator is that it's limited to 30 values, http://stackoverflow.com/questions/3825845/computing-number-of-sub-queries-in-google-appengine , and we'll probably see an hfeed in the wild with >30 entries before too long. so, we probably need to split up the urls list, generate a query for each one, and return a chained iterator over those queries. itertools might make this not too hard?