Add the ability to reset "ep_item_commit_failed" and "ep_item_flush_failed" stats via cbstats

cbstats reset does not reset stats for "ep_item_commit_failed" and "ep_item_flush_failed" and currently, the only way to reset these stats is to restart couchbase-server. Restarting nodes is too heavy-handed for the sole purpose of clearing stats.

As some monitoring systems may report false alarms due to these values staying non-zero after resolving the underlying issue, it would be useful to be able to clear these stats with cbstats reset.