raft.c: Stale leader should disconnect from cluster.

raft.c: Stale leader should disconnect from cluster.

As mentioned in RAFT paper, section 6.2:

Leaders: A server might be in the leader state, but if it isn’t the current
leader, it could be needlessly delaying client requests. For example, suppose a
leader is partitioned from the rest of the cluster, but it can still
communicate with a particular client. Without additional mechanism, it could
delay a request from that client forever, being unable to replicate a log entry
to any other servers. Meanwhile, there might be another leader of a newer term
that is able to communicate with a majority of the cluster and would be able to
commit the client’s request. Thus, a leader in Raft steps down if an election
timeout elapses without a successful round of heartbeats to a majority of its
cluster; this allows clients to retry their requests with another server.

Reported-by: Aliasgar Ginwala <aginwala@ebay.com>
Tested-by: Aliasgar Ginwala <aginwala@ebay.com>
Signed-off-by: Han Zhou <hzhou8@ebay.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>