Figure out if caching locality lb endpoints breaks xDS protocol

During development of https://github.com/allegro/envoy-control/issues/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)