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.