Add rib subscription API.

[melifaro]

Summary

Currently there is no easy way of subscribing for the routing table changes. The only existing way is to set ifa_rtrequest callback in the each protocol ifaddr, which is not convenient or extandable.

This change provides generic notification subscription mechanism, that will replace current ifa_rtrequest one and allow other applications such as accelerated routing lookup modules subscribe for the changes.

In particular, this change provides 2 hooks: 1) synchronous one (RIB_NOTIFY_IMMEDIATE), called under RIB_WLOCK, which ensures exact ordering of the changes and 2) async one, (RIB_NOTIFY_DELAYED) that is called after the change w/o holding locks. The latter one does not provide any notification ordering guarantee.

This change depends on D25067.

[melifaro]

Add forgottech epoch reclamation.

[ae]

How many subscribers do you expect? I think you will replace some existing. Maybe it would be better to have separate list for each subscription type?

[melifaro]

In D25070#552529, @ae wrote:

How many subscribers do you expect? I think you will replace some existing. Maybe it would be better to have separate list for each subscription type?

Currently there is only one - ND6 default route checker. With the modular lookup engine, there will be 1 more for rtables where lookup engine is attached.
With that in mind, idea is to start with the simplest implementation and then enhance it if/when needed.