private CreateVisitorReply createReply(CreateVisitorMessage msg) {
        CreateVisitorReply reply = (CreateVisitorReply)msg.createReply();
        reply.setMessage(msg);
        return reply;
    }
Mock the executor to keep things nicely single threaded for the testing.
No need to synchronize things here since we don't use multiple threads.
documentapi/src/test/java/com/yahoo/documentapi/messagebus/MessageBusVisitorSessionTestCase.java