Fix bug where stats endpoints were calculating _all_ riak_kv stats

Since adding many more stats, and most of the infrastructure for
ad hoc querying of stats, the stat calculation code for the
(not yet legacy) endpoints was calculating all stats for riak_kv.

As there are about (ring_size * vnode stats) + (fsm stages * fsm stats)
more stats now, this calculation, understandbly, took a long time.

This patch instead only calculates the minimum subset of stats
needed to support the (not yet legacy) stats endpoints.