Turbo top

Further refinements to the table/:name/top query
* make it faster by using Stats() instead of iteration over all keys
* add more stats to the results:
** Inuse: reports the total byte size used for object's events
** Alloc: reports the allocated byte size for the object (0 for inlined objects since they use parent bucket's allocation)
* add another query parameter `by`, that allows switching between `size` and `count` as the measurment criteria (default `size`), where `size` means the Inuse value

@benbjohnson 
/cc @snormore @charlescng