checkHost check the host or IP address is valid and available.

CopyStringMap copy the src map's key and value, and return the dst map.

CopyStringSet copy the src map's key, and return the dst map.

GetConfirmedPassword gets double confirmed password from user input.

GetPassword gets password from user input.

newHistory creates a new history instance.

HeightChanges holds all changes on a particular height.

History is a helper to log all producers and votes changes for state, so we can handle block rollback by tracing the change history, no need to loop though all transactions since the beginning of DPOS consensus.