RCBC-151 Return CAS in extended mode for incr/decr

Change-Id: I1ad55b55f5828858c1a83eb30a7ce88f92609c53
Reviewed-on: http://review.couchbase.org/31243
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>