NCBC-293: enhance or create new multiget to allow getting details on
missing items.

Note this is a resubmit of http://review.couchbase.org/#/c/29991/ which
became lost in some expliciable way. To make sure it's in, this change
was cherry-picked over current and fixed up.

Change-Id: I6c8d397f76c6079d501e390844b55eb72b7a2bd1
Reviewed-on: http://review.couchbase.org/30708
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>