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 I added some comments and restructured the method to be clearer. There is already a TODO to add support for updated URLs [in tasks.refetch_hfeed](https://github.com/snarfed/bridgy/blob/master/tasks.py#L320) if that is good enough.

And actually it should be OK for there to be multiple syndication URLs (even for the same Source) pointing to the same original... Like in the case of syndicating a longer post to a pmarca-style tweetstorm. So I was wrong to say that we shouldn't ever be there with `this original -> different syndication`