Asynchronous (de-)serializers

[Airblader]

What's your idea?

(De-)serializers are currently synchronous, but it could make sense to have them be async. This we should consider allowing this.

Describe the solution you'd like

Have them return either a synchronous value or an observable.

Additional context

We need to investigate and discuss this first as this might introduce some questions.

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Update 2019/04/26: Asynchronous deserializers are implemented now.

[Airblader]

If implemented, we should explicitly define & document whether we react on first emission or completion of the returned observable.

[Airblader]

I think the biggest use-case here is deserializers being asynchronous, so we could also focus on only making those asynchronous for now.

For serializers due to CVAs not being reactive we could run into race conditions that we cannot avoid from ngqp.