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/remove_command.go

@@ -68,6 +68,7 @@ func applyRemove(c *RemoveCommand, context raft.Context) (uint64, error) {
 		} else {
 			// else ignore remove
 			log.Debugf("ignore previous remove command.")
+			ps.removedInLog = true
 		}
 	}
 	return commitIndex, nil