async / await works for Ask, and have done so since very early versions.
(but only for Ask since we could force the response to actually be handled this way)

e.g.
```csharp

override void OnReceive(...)
{
     if (..)
        DoStuff();
        //OnReceive will exit once the await clause is started
}
private async void DoStuff()
{
        //method will return here, and OnReceive will exit
        //making it possible for other messages to arrive in OnReceive
        var res = await otherActor.Ask(...); 

         //using the old code, this would be guaranteed to never run at the same time as 
         //OnReceive..the new code does not prevent this

         this.MutableField ++; //or other non threadsafe action
}
```

If the `CompleteFuture` is handled inside the mailbox, then we can guarantee that no other message is beeing processed by the actor and thus prevent any threading issues.