MB-11554: Keep cas during recovery

Pass through original cas value which is read from source

Change-Id: I1b357f8c5b360db0e13e6f7e11abdec5b07ffdab
Reviewed-on: http://review.couchbase.org/39323
Tested-by: Bin Cui <bin.cui@gmail.com>
Reviewed-by: Bin Cui <bin.cui@gmail.com>