ospf6d: Fix ECMP for asbr external routes

ospf6d: Fix ECMP for asbr external routes

Use brouter table to fetch nexthops for
asbr prefix (external) routes.
Change adv. router of the router's path once
the DB/FIB is updated with effective nexthops.
Cleanup of nexthop update when route's adv
router changes cost.

Ticket:CM-16139
Testing Done:
Tested ASBR external routes in CLOS topology with
multiple paths asbr originator at tor to spine.
Validated external route's nexthop within
area and inter area.

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>