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)
i haven't fully followed the logic here yet, but i'm curious what will happen if we have an existing (non-blank) SyndicatedPost where one of the values is the same, but the other is different. i'm guessing we'll leave it, and not store the new relationship?

i haven't thought through what we *should* do in that case, but i'm guessing we should allow overlapping entities like that, but also notice when an existing one we have is removed from an h-entry. the concrete case would be you posse to twitter, we grab your rel-syndication link, then later you notice that your possed tweet had a typo, so you delete it, post a new correct one, and update your rel-syndication link.

doesn't have to be handled in this PR, but worth thinking about.