CBIDXT-174 Store Stability Timestamp in Snapshots

Stability Timestamp is based on UPR snapshots.

Change-Id: Iac663eb207e7eedf46d0c38d5e1f3b3ca0e40d5b
Reviewed-on: http://review.couchbase.org/42883
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>