[FEATURE-REQ]update node public key after registration done

[Honglei-Cong]

Is your feature request related to a problem? Please describe.

Ontology node owner provides his node public key when registering. If he wants to change his node public key, he'll have to unregister the old one, and register a new Ontology node currently. But unregister/register both will take more than one consensus epoch.

Describe the solution you would like

Updating node public key, rather than unregister-register Ontology node.