feat: make the observers asynchronous

[ziccardi]

Observers are currently synchronous: a slow observer would slow down the whole routine manager.
This PR makes the observers asynchronous

[tzvatot]

Couple of thoughts:

1. We don't nervelessly have a requirement to run the observers async. While in some cases it make sense, and has value, the consumer of the framework can achieve that by creating a top level observer, that will notify a set of child-observers asynchronously. So I'm contemplating between keeping the framework simple and yet expandable, than introducing a level of complexity that we don't necessarily need, which also adds some risk.
2. This PR does not have any tests attached. Given #1, maybe that is not required.