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)
minor, optional: a dict comprehension can simplify this a bit if you want: `preexisting = {r.original: r for r in SyndicatedPost.query_by_originals(source, permalinks)}`