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)
thanks!

so, i'm still thinking through the logic and all possible cases here. it sounds like this is a fully many-to-many relationship, ie we expect that we'll see some SyndicatedPosts with the same original and different syndications (use case: pmarca), and we also expect some with the same syndication and different originals (use case: [amending a previous post](http://indiewebcamp.com/irc/2014-06-19/line/1403207765)).

given that, i suspect we need to fully replace `query_by_original` with `query_by_originals`, and same with syndication, and change all callers to handle (usually iterate over) multiple results. does that sound right?

apologies if this is all scope creep for this PR. i'm honestly not sure myself. :P

also, minor: consider renaming this method `get_or_insert_with`, or something similar, since it provides both original and syndication.