private void tryShutdown() {
        if (isClosing() && isOkToClose())
            shutdown(closeFuture.get());
    }
Close was signaled in closeAsync() but there were pending messages at that time. This method attempts the
shutdown if the returned result cleared up the last pending message.
gremlin-driver/src/main/java/org/apache/tinkerpop/gremlin/driver/Connection.java