Refactor `pkg/proxy/config`’s ServiceConfigHandler.OnUpdate and
EndpointsConfigHandler.OnUpdate to different method names as they have
different signatures.
This will let the new proxy
(https://github.com/GoogleCloudPlatform/kubernetes/issues/3760)
implement both interfaces.
Since we won’t need a separate loadbalancer structure (load balancing
is handled in the proxy rules), we will simply handle both event types
from the same object.
