Add basic stats for strongly consistent operations

Consistent gets/puts now correctly increment `vnode_gets`/`vnode_puts` the same as normal gets/puts. Prior to this commit, gets incremented the relevant stat but puts did not.

Consistent operations now maintain per-operation stats that are similar to the node/FSM stats for normal gets/puts. This includes operation counts, latency percentiles, and object size percentiles.

Future work is needed to support per-bucket stats, detailed `riak_ensemble` stats that track FSM stages / interesting events (eg. object rewrites after leader change), etc. But, these basic stats should be good enough for 2.0.

Addresses #876 

in other places this has led to negative numbers being passed to folsom, which silently writes them and later dies on calculation: https://github.com/basho/riak_kv/issues/452. Not sure if we want to address it here, or hope that folsom/riak_kv_stat is patched up soon.