Align async events with EDR1

Make sure the async event implementation matches what's defined in EDR1, namely:

• make a distinction between a synchronous and asynchronous (@ObservesAsync) observers
• fire() should only notify sync observers
• fireAsync() should notify sync observers first and then async ones in a worker thread
• use FireAsyncException
• keep current async notification semantics (observers notified serially in a single worker thread) but add special exception handling (an exception does not abort the async chain)