SubnetRoute re-election must happen when the anchor switch for subnetRoute remains disconnected.

More summarily:
Subnet Route reelection algorithm is triggered today only when
the anchor ports on the subnet go to DOWN state (or) if such
anchor ports on the subnet are being removed.

However, there are situations where a switch hosting the anchor
port can be disconnected on the openflow control-path and can
continue to remain so indefinitely.

Under such circumstance, new non-neutron IPs become undiscoverable
as the switch hosting the anchor port cannot punt discoverable IP
packets to the controller.

So this fix attempts to always trigger reelection of nodes for
subnetRoute hosting as soon as the current anchor node(or switch)
is detected to be disconnected from the controller.