CBD-648: Delete algorithm for vtree

The deletion algorithm is similar to the insertion algorithm,
it uses the functionality from vtree_modify, just as
vtree_insert does.

It also support bulk deletes.

Change-Id: Ie4453fa3b2c16ea214490a12e48a9f3a215fd43d
Reviewed-on: http://review.couchbase.org/22335
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>