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.
This commit adds a method to the `LoadBalancer` interface in the
userspace proxy which allows consumers of the `LoadBalancer` to check if
it thinks a given service has endpoints available.
Moves the userspace code in proxy to a sub-package and adds the
ProxyProvider interface.
This is in preparation for landing an implementation of
https://github.com/GoogleCloudPlatform/kubernetes/issues/3760, which
will mostly be in another sub package for iptables.
