CBD-648: Move within_mbb/3 to vtree_util

This is a commit in preparation for the deletion algorithm that
needs the within_mbb/3 function. It's a no-functional change that
just moves the within_mbb/3 function around.

Change-Id: I18e1693fcba48afd12a3fbd12416c4e7c7418c71
Reviewed-on: http://review.couchbase.org/22338
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>