When deleting a peering we do not want to delete the peering and all
imported data in a single operation, since deleting a large amount of
data at once could overload Consul.
Instead we defer deletion of peerings so that:
1. When a peering deletion request is received via gRPC the peering is
marked for deletion by setting the DeletedAt field.
2. A leader routine will monitor for peerings that are marked for
deletion and kick off a throttled deletion of all imported resources
before deleting the peering itself.
This commit mostly addresses point #1 by modifying the peering service
to mark peerings for deletion. Another key change is to add a
PeeringListDeleted state store function which can return all peerings
marked for deletion. This function is what will be watched by the
deferred deletion leader routine.
freddygv