Simplify async use in subscribers

[sandtreader]

Currently if a subscriber uses the MessageBusExt sugar function subscribe(), the handler gets called as a sync lambda, which means it has
to use tokio::spawn() to be able to call the async publish, which is by far the most common pattern.

We need a slightly less sugared version of subscribe() where you can subscribe with an async lambda. If this works well, it might be worth making this the default and taking a flag day hit on all existing modules.

[sandtreader]

Done as described, but now we have async closures, this could be simplified still further