CBD-65: Make vtree_split:split_axis/4 return the dimension

For forther steps in the algorithm we need to know on which
dimension the data was split for the resulting split axis.

Change-Id: Ia0c3f3d104d8155a96343beb6eddebfffbb1e149
Reviewed-on: http://review.couchbase.org/21019
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>