Enhance cluster start/restart for administration

DG7 included the DomainMode where a separate process handled the configuration and server restarts and monitoring.

This feature is lost in DG8 as it does not longer inherit the EAP server.

There should be a possibility to start a cluster and prevent from any client access until an administrator will 'enable' it with a single CLI action.

Also the internal state-transfer and pre-load should optimized for this