func (r *Raft) RemoveServer(id ServerID, prevIndex uint64, timeout time.Duration) IndexFuture {
	if r.protocolVersion < 2 {
		return errorFuture{ErrUnsupportedProtocol}
	}

	return r.requestConfigChange(configurationChangeRequest{
		command:   RemoveServer,
		serverID:  id,
		prevIndex: prevIndex,
	}, timeout)
}
func (r *Raft) RemoveServer(id ServerID, prevIndex uint64, timeout time.Duration) IndexFuture {
	if r.protocolVersion < 2 {
		return errorFuture{ErrUnsupportedProtocol}
	}

	return r.requestConfigChange(configurationChangeRequest{
		command:   RemoveServer,
		serverID:  id,
		prevIndex: prevIndex,
	}, timeout)
}
// RemoveServer will remove the given server from the cluster. If the current
// leader is being removed, it will cause a new election to occur. This must be
// run on the leader or it will fail. For prevIndex and timeout, see AddVoter.