bgpd: L3NHG infrastructure for host routes in EVPN

bgpd: L3NHG infrastructure for host routes in EVPN

ES-VRF entries are maintained for the purpose of L3-NHG creation -
1. Each ES-EVI entry is associated with a tenant VRF. This associaton
triggers the creation of an ES-VRF entry.
2. Type-2/MAC-IP routes are imported into a tenant VRF and programmed as
a /32 or host route entry in the dataplane. If the destination of
the host route is a remote-ES the route is programmed with the
corresponding (keyed in by {vrf,ES-id}) L3-NHG.
3. The reason for this indirection (route->L3-NHG, L3-NHG->list-of-VTEPs)
is to avoid route updates to the dplane when a remote-ES link flaps i.e.
instead of updating all the dependent routes the NHG's contents are
updated. This reduces the amount of dataplane updates (fewer nhg updates vs.
route updates) allowing for a faster failover.

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