This method uses raft status exposed at /debug/varz to determine the
health of the cluster. It uses whether commit index increases to
determine the cluster health, and uses whether match index increases to
determine the member health.
This could fix the bug #2711 that fails to detect follower is unhealthy
because it doesn't rely on whether message in long-polling connection is sent.
This health check is stricter than the old one, and reflects the
situation that whether followers are healthy in the view of the leader. One
example is that if the follower is receiving the snapshot, it will turns
out to be unhealthy because it doesn't move forward.
`etcdctl cluster-health` will reflect the healthy view in the raft level,
while connectivity checks reflects the healthy view in transport level.
When failing to get leader stats, it said 'cluster is unhealthy' before.
This is confusing when it cannot get stats because advertised client urls
are set wrong and the cluster is healthy.
etcdctl reconnects to the leader, but was not picking up ssl settings in this
case, which causes it to show unhealthy when this is not the case.
Fixes#2373
