feat: implement standby mode

Change log:
1. PeerServer
- estimate initial mode from its log through removedInLog variable
- refactor FindCluster to return the estimation
- refactor Start to call FindCluster explicitly
- move raftServer start and cluster init from FindCluster to Start
- remove stopNotify from PeerServer because it is not used anymore
2. Etcd
- refactor Run logic to fit the specification
3. ClusterConfig
- rename promoteDelay to removeDelay for better naming
- add SyncClusterInterval field to ClusterConfig
- commit command to set default cluster config when cluster is created
- store cluster config info into key space for consistency
- reload cluster config when reboot
4. add StandbyServer
5. Error
- remove unused EcodePromoteError

server/client.go

@@ -30,7 +30,7 @@ func NewClient(transport http.RoundTripper) *Client {
 	return &Client{http.Client{Transport: transport}}
 }
 
-// CheckVersion checks whether the version is available.
+// CheckVersion returns true when the version check on the server returns 200.
 func (c *Client) CheckVersion(url string, version int) (bool, *etcdErr.Error) {
 	resp, err := c.Get(url + fmt.Sprintf("/version/%d/check", version))
 	if err != nil {