Collections: topkeys should track the default collection only

topkeys returns a JSON object with information about which keys are 'hot', the JSON object returned has no collection field and the code itself is not collection aware, allowing for key1 from different collections to contribute to a single counter.

We need

1) non collection-aware client should get topkeys in the 'old' format and only default-collection keys.
2) collecton-aware client gets JSON doc with new field
3) topkeys code must track keys with collection awareness, i.e. don't allow keys to collide.