MB-100: always do sequential multiGet

Otherwise it fails on old envs. and causes extra delay

Change-Id: I44d047f0b9e32fcda4a05ad44695d977320f006c
Reviewed-on: http://review.couchbase.org/24550
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>