Async notifications

[wilzbach]

listen, trigger

[wilzbach]

The following works now:

subscriber.subscribe("test1", "test2");
auto task = subscriber.listen((string channel, string message) {
    writefln("channel: %s, msg: %s", channel, message);
});

conn.publish("test1", "Hello World!");
conn.publish("test2", "Hello from Channel 2");

runTask({
    subscriber.subscribe("test-fiber");
    subscriber.publish("test-fiber", "Hello from the Fiber!");
    subscriber.unsubscribe();
});

channel: test1, msg: Hello World!
channel: test-fiber, msg: Hello from the Fiber!
channel: test2, msg: Hello from Channel 2