protected void responseAvailable() {
        if (monitor != null) {
            monitor.responseAvailable(this);
        }
    }
@param query the query to send
@param context a context object that can be used to pass context among different
               invokers, e.g for reuse of preserialized data.
@return an object that can be passed to the next invocation of sendSearchRequest
container-search/src/main/java/com/yahoo/search/dispatch/SearchInvoker.java