bgpd: Use L3NHGs for symmetric IRB host routes

bgpd: Use L3NHGs for symmetric IRB host routes

Two L3 next groups are installed per-VRF per-ES for v4 and v6. These
NHGs are used as an indirect destination for symmetric IRB host routes.

Using L3NHGs allows for efficient failover of an ES (similar to the
use of L2NHGs) i.e. when an ES goes down the number of dataplane
updates are limited to 2xN (where N is the number of tenant VRFs
associated with the ES) instead of updating all host-routes behind the
ES.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>