MB-100: Move calc_mbb/2 to vtree_util

Change-Id: I67ad0b1a6cc318163f023072493e72e26b0ca14d
Reviewed-on: http://review.couchbase.org/21022
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>