bgpd: convert NHT code to use rb-trees instead of routing tables

bgpd: convert NHT code to use rb-trees instead of routing tables

Fist, routing tables aren't the most appropriate data structure
to store nexthops and imported routes since we don't need to do
longest prefix matches with that information.

Second, by converting the NHT code to use rb-trees, we can index
the nexthops using additional information, not only the destination
address.  This will be useful later to index bgpd's nexthops by
both destination and SR-TE color.

Co-authored-by: Sebastien Merle <sebastien@netdef.org>
Signed-off-by: Renato Westphal <renato@opensourcerouting.org>