NetworkKeyProvisioning

Receives this backend's network key from the proxy.

A backend cannot derive the key on its own — that was the flaw in the previous design — and it cannot ask for one either: its only route to the proxy is a clientbound plugin message, which proxies drop for any channel the player's client has not registered. So the proxy pushes instead, as soon as a player connects here. The key is persisted, making this a once-per-backend event.

Grants are only honoured while this backend holds no key. See acceptProvisionedKey.