proxy: consolidate ServicesHandler/EndpointsHandler into ProxyProvider

Proxies should be able to cleanly figure out when endpoints have been synced,
so make all ProxyProviders also implement EndpointsHandler and pass those
through to loadbalancers when required.

pkg/proxy/userspace/loadbalancer.go

@@ -19,6 +19,7 @@ package userspace
 import (
 	"k8s.io/api/core/v1"
 	"k8s.io/kubernetes/pkg/proxy"
+	proxyconfig "k8s.io/kubernetes/pkg/proxy/config"
 	"net"
 )
 
@@ -31,4 +32,6 @@ type LoadBalancer interface {
 	DeleteService(service proxy.ServicePortName)
 	CleanupStaleStickySessions(service proxy.ServicePortName)
 	ServiceHasEndpoints(service proxy.ServicePortName) bool
+
+	proxyconfig.EndpointsHandler
 }