rb_tree: Add search helper function

Find the node before you delete it

Signed-off-by: Prashant Malani <p.malani@gmail.com>
rb_tree: Add "delete" initial removal

Implement the first part of delete, where we
splice out a node and reconnect the tree.

What remains is to perform the fixup(rebalancing)

Also fix a whole in insert code pertaining to root
update.

Signed-off-by: Prashant Malani <p.malani@gmail.com>