feat: implement standby mode

Change log:
## 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

## Etcd

 - refactor Run logic to fit the specification

## 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

## add StandbyServer

## Error

 - remove unused EcodePromoteError

Split from #764.
Have made modification based on the discussion in this issue.