Async event not CompletionStage friendly

The goal of this ticket is to enable user to get injected the completion future instead of the raw event and return another completion stage (generic type ignored) to enable the "chain" to be synched.

Here is a sample:

1. fireAsync(persistEvent)
2. observer implementation does: return future.thenCompose(db::doAsyncPersist)
3. when the user gets back the result of the fireAsync the doAsyncPersist is completed.