Ask Pattern

[ognen]

Implement the ask pattern in ActorRef. The basic signature is already there.

Implementation details. Whenever someone asks an actor about something, the following happens:

• a temporary actor is created under /system/queries
• this actor poses the question and awaits the answer (for a certain amount of time)
• once the response msg arrives it completes the callback
• on timeout, it should complete with error.

Here in the callback signature, it's OK to use Swift 5's result type.