Add Yokozuna stats so they appear in console and http /stats

[bowrocker]

Yokozuna stats are current not appearing in 'riak-admin status' or http output. Add them to riak_kv_status and riak_kv_wm_stats so that they do.

[rzezeski]

Testing both riak-admin status and /stats manually and both work.

# curl -s 'http://localhost:10018/stats' | json_pp | grep search
   "search_query_latency_max" : 0,
   "search_query_latency_min" : 0,
   "search_query_latency_95" : 0,
   "search_index_latency_median" : 0,
   "search_index_fail_count" : 0,
   "search_index_latency_99" : 0,
   "search_index_throughtput_one" : 0,
   "search_index_latency_999" : 0,
   "search_index_latency_min" : 0,
   "search_query_throughput_one" : 0,
   "search_index_latency_max" : 0,
   "search_query_latency_median" : 0,
   "search_query_latency_999" : 0,
   "search_query_throughput_count" : 0,
   "search_index_throughput_count" : 0,
   "riak_search_version" : "1.4.1-22-ga3893df",
   "search_query_latency_99" : 0,
   "search_index_latency_95" : 0,
   "search_index_fail_one" : 0,

#  ~/yz-verify/rt/riak_yz/dev/dev1/bin/riak-admin status | grep search
riak_search_version : <<"1.4.1-22-ga3893df">>
search_query_throughput_count : 0
search_query_throughput_one : 0
search_query_latency_min : 0
search_query_latency_max : 0
search_query_latency_median : 0
search_query_latency_95 : 0
search_query_latency_99 : 0
search_query_latency_999 : 0
search_index_throughput_count : 0
search_index_throughtput_one : 0
search_index_fail_count : 0
search_index_fail_one : 0
search_index_latency_min : 0
search_index_latency_max : 0
search_index_latency_median : 0
search_index_latency_95 : 0
search_index_latency_99 : 0
search_index_latency_999 : 0

+1 to merge.

[mbbroberg]

Thank you for this PR @bowrocker! It's great to see it merged. 👍