private synchronized void createQueueIfNotExists(GetConfigContext context) {
        if ( ! delayedResponses.containsKey(context.applicationId())) {
            delayedResponses.put(context.applicationId(), new LinkedBlockingQueue<>());
        }
    }
Creates a DelayedConfigResponse object for taking care of requests that should
not be responded to right away.  Puts the object on the delayedResponsesQueue.

NOTE: This method is called from multiple threads, so everything here needs to be
thread safe!

@param request a JRTConfigRequest
configserver/src/main/java/com/yahoo/vespa/config/server/rpc/DelayedConfigResponses.java