Agent re-registration

[kate-osborn]

On a controller restart event, all connected agents will re-connect to the controller but will not re-register. This makes all previously connected agents "unconfigurable" because the controller does not have the registration bits (system ID and nginx ID) required to push config to the agents. NKG needs the agent to re-register with the controller on every connection. Once this feature is added to the agent, we can test and close this issue.

Parent issue: #375

Aha! Link: https://nginx.aha.io/features/NKG-53