Optional<RawConfig> resolveConfig(JRTServerConfigRequest req) {
        // Calling getConfig() will either return with an answer immediately or
        // create a background thread that retrieves config from the server and
        // calls updateSubscribers when new config is returned from the config server.
        // In the last case the method below will return empty.
        return configClient.getConfig(RawConfig.createFromServerRequest(req), req);
    }
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