Rather than calculate stats on demand when stale, backfill the cache

Always server the stats that are in the cache, no matter how old they are.
Add a timestamp to the stats so consumers know how stale they are.
Fill the cache continuously in the background.

See https://github.com/basho/riak_kv/issues/505 and https://github.com/basho/riak_kv/issues/506 for graphs, bench configs etc.