Figure out if caching locality lb endpoints breaks xDS protocol

[slonka]

During development of #10 we stumbled upon an issue of caching (see related issue for full story). We should investigate if it breaks https://github.com/envoyproxy/data-plane-api/blob/master/XDS_PROTOCOL.md and report that to java-control-plane if it does. Also our fix for this issue is to not remove a service completely from serviceNameToInstances map - that would cause envoy to return 503 (no instances in a cluster) instead of 404 (no cluster)

[Automaat]

I have checked it and it breaks xDS protocol. It is mostly problematic for services with * in dependencies. When we have * in dependencies we try to extract snapshot configuation for services that does not exist any more but are present in envoy-control ServicesState. And if we cannot extract this config Envoy cannot fully start. I think that because we don't remove services from serviceNameToInstances map there was neeed to copy SimpleCache from java-control-plane and add custom logic to sent EDS config for not existing services.