zebra: Abstract the RB nodes/add dependents tree

zebra: Abstract the RB nodes/add dependents tree

Create a nhg_depenents tree that will function as a way
to get back pointers for NHE's depending on it.

Abstract the RB nodes into nhg_connected for both depends and
dependents. This same struct is used for both.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>