bgpd: add addpath ID to adj_out tree sort

bgpd: add addpath ID to adj_out tree sort

When withdrawing addpaths, adj_lookup was called to find the path that
needed to be withdrawn. It would lookup in the RB tree based on subgroup
pointer alone, often find the path with the wrong addpath ID, and return
null.  Only the path highest in the tree sent to the subgroup could be
found, thus withdrawn.

Adding the addpath ID to the sort criteria for the RB tree allows us to
simplify the logic for adj_lookup, and address this problem. We are able
to remove the logic around non-addpath subgroups because the addpath ID
is consistently 0 for non-addpath adj_outs, so special logic to skip
matching the addpath ID isn't required.  (As a side note, addpath will
also never use ID 0, so there won't be any ambiguity when looking at the
structure content.)

Signed-off-by: Mitchell Skiba <mskiba@amazon.com>