Allow to define a listener to be executed asynchronously

It is currently possible to define custom listener for various kind of events, but apparently those listeners are always executed synchronously by the Observation Manager. We can of course implement them so they perform their task in a new thread once they are called, but it looks like a generic need to be able to define a Listener that would be executed asynchronously.

For example, we have a listener executed in case of page creation that needs to communicate with another server. It currently slows down a lot the page creation because it's executed synchronously: we could benefit directly of having an asynchronous execution of this listener by default.