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)
posse-post-discovery: limit IN query to 30 items
posse-post-discovery: clarify SyndicatedPost.get_or_insert

Restructed get_or_insert_by_syndication_url to be much clearer
about what happens in the different cases, and added comments to
the same effect.
Merge pull request #199 from kylewm/ppd-optimize-deletes

posse-post-discovery: reduce number of NDB operations