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)
Good point. Right now the refetch behavior explicitly only tries to update blanks...does not support changes yet. Sigh :)

> 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.

Differs depending on which value is new.

If there is an existing `this original -> different syndication` relationship, a) we should not be in this method because there is an earlier check for a preexisting relationship that should have prevented it, and b) if we somehow are here, this would add a new SyndicatedPost for the same original URL.

If there is an existing `different original -> this syndication` relationship, this method would not save a new one.