MB-8825: Add stat to keep track of conflict resolution fails

This stat will help with xdcr debugging

Change-Id: Ie2ea77f51f40581ab60c823cf40c5276b4e27845
Reviewed-on: http://review.couchbase.org/28271
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>