Add support for replica get with CAS (ReplicaGets).

This changeset adds the underlying operations and clone-logic for
replica get operations including the underlying CAS value.
This can be utilized by the upper layers to provide responses that
not only include the raw value from a replica, but also their
corresponding CAS value.
Change-Id: Ib218669b6bbc84f3111e107e14761a03ed2f9728
Reviewed-on: http://review.couchbase.org/33799
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>