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)
ok, thanks. i'm fine with just adding a TODO, or even just a comment if we're happy with the current behavior for now. we can file an issue to eventually support changed rel-syndication urls in the future if we want.

btw, if you're pretty confident that we should never get here if there's an existing `this original -> different syndication` relationship, then maybe add an assert?