MB-100: multi get/set xdcr optimizations

Change-Id: Ia6e12e4fe905d6cc6360a0fee48a630de2126f47
Reviewed-on: http://review.couchbase.org/24679
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>