Various fixes to pass the verify_stats r_t test case

- Checks for an ok return from all exometer:update calls in riak_kv_stat
- Removes automated repair logic that attempted in riak_kv_stat to re-register a metric
  whenever an exception was thrown.  This approach could result in an
infinite loop if the exception was not caused by a missing metric.
- Adds explict calls to create_or_update to riak_kbv_stat when using
  dynamically created metrics
- Fixes internal metric labels in riak_kv_wm_stats for counters and stat
  times