zebra: support for slow-failover of local MACs on an ES

zebra: support for slow-failover of local MACs on an ES

When a local ES flaps there are two modes in which the local
MACs are failed over -
1. Fast failover - A backup NHG (ES-peer group) is programmed in the
dataplane per-access port. When a local ES flaps the MAC entries
are left unaltered i.e. pointing to the down access port. And the
dataplane redirects traffic destined to the oper-down access port
via the backup NHG.
2. Slow failover - This mode needs to be turned on to allow dataplanes
not capable of re-directing traffic. In this mode local MAC entries
on a down local ES are re-programmed to point to the ES-peers'
NHG. And vice-versa i.e. when the ES comes up the MAC entries
are re-programmed with the access port as dest.

Fast failover is on by default. Slow failover can be enabled via the
following config -
evpn mh redirect-off

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