Store entry responses.

Store responses fetched by async log client, and use them when available. Since we aren't using unsafe pickle I've set use_stored_entries flag to true by default. 
However, storing one file per EntryResponse might not be the best aproach.
Nit: Extract this to a separate method so the flow of _fetch_parsed_entries is not cluttered with DB-specific logic. Something like:

stored_entries = self._get_entries_from_db(first, last)
if stored_entries:
  stored = defer.Deferred()
  ...
  return stored
...