feat(standby_server): write cluster info to disk

For better fault tolerance and availability.
docs(standbys): clarify pseudocode
chore(standby_server): still sleep for the first time
feat(standby_server): save/load syncInterval to disk
feat(standby_server): save/load Running into disk
feat(standby_server): make atomic move for file

to avoid the risk of writing out a corrupted file.
chore(standby_server): remove error return

because standby server should be started in best efforts.
Merge pull request #786 from unihorn/91

feat(standby_server): write cluster info to disk