MB-10065: Use real CAS value for mutations

XDCR uses CAS as part of the revision, hence it's important
to have the correct value.

Change-Id: Ib40d80f90e3df977de23424cd1e9848a85d5ad74
Reviewed-on: http://review.couchbase.org/33035
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>