Implement ASPA support in RTR by following draft-ietf-sidrops-8210bis-10.

Implement ASPA support in RTR by following draft-ietf-sidrops-8210bis-10.

In rtr.c renamed rtr_aspa_merge_set() to rtr_aspa_insert() and move it
close to rtr_roa_insert().
In rtr_proto.c most complexity comes from the version negotiation. The
ASPA parser is reasonably streight forward. The version negotiation is
fragile but that is mostly because of the protocol specification and the
fact that RTR cache daemons sometimes fail to send errors.
OK tb@