LWW/revId buckets to custom bucket migration

When a bucket is upgraded to custom CR, we need to avoid unnecessary merges for the existing documents during backfill.
Documents create/updated before bucket CR upgrade should continue to use old CR method avoid backfill performance hit. To differentiate documents before/after upgrading to custom CR, we can check if document CAS is higher than the upgrade timestamp. We will need a REST API to fetch target CR upgrade timestamp.To avoid merging with predecessor, one option is the mutation with xattrs wins.