private ConfigProxyRpcServer createRpcServer(Spec spec) {
        return  (spec == null) ? null : new ConfigProxyRpcServer(this, supervisor, spec); // TODO: Try to avoid first argument being 'this'
    }
A proxy server that handles RPC config requests. The proxy can run in two modes:
'default' and 'memorycache', where the last one will not get config from an upstream
config source, but will serve config from memory cache only.

@author hmusum
config-proxy/src/main/java/com/yahoo/vespa/config/proxy/ProxyServer.java