implement proper ECMP route handling

[KanjiMonster]

Implement ECMP route handling:

• use set -> ecmp group id matching, for stable groups regardless of reachability of nexthops
• make nh_{un}reachable_notification update ecmp routes, and not just mark them as (un)routable

TODO:

• reuse old ecmp ids like we do for l3 interface ids (to avoid running out of ids) - may uncover bugs
• more testing