MB-8214: Add xdcr conflict resolution to ep-engine

This change adds the xdcr conflict resolution mechanism
to ep-engine and allows setMeta, addMeta, and delMeta to
do background fetches in order to decide whether or not we
should persist and item sent ot us by xdcr.

Change-Id: I8a94432b1179511a632a03e9cb73ccba10389119
Reviewed-on: http://review.couchbase.org/26422
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>