keys_only=True queries count as "small datastore ops".

Under certain scenarios, many of the entities returned from a query may already 
be in cache - in particular memcache.

It would be great if the NDB framework could automatically convert a query to 
keys_only=True and then a get_multi() using the returned keys so that the 
entities would be returned from the caches (of course, only if present).

For queries that return frequently accessed entities, this could end up being a 
significant cost (and performance?) savings and could all be provided under the 
hood by NDB.

The API might be as simple as:

  Entity.query(use_cached_entities=True)

Of course, the following would raise some kind of exception because it's 
inconsistent:

  Entity.query(use_cached_entities=True, keys_only=False)