Add tables/:name/objects/stats?top=X endpoint

This is meant to help finding anomalous large objects (currently meaning objects with large event counts). The endpoint allows specifying how many top objects you want to see, defaults to 100.

The implementation is fairly straightforward, iterates over each object counting the events and keeping an ordered slice of top id/count pairs, which gets returned at the end. The event counting is naive, but there doesn't seem to be faster way to get key count out of bolt at this point.

@benbjohnson 

/cc @snormore @charlescng