Async Updates

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Currently, all updates to values in the store happen synchronously.

While this leads to mostly predictable behavior overall, I feel like updates should happen asynchronously. This is mostly for performance reasons.

I think, React developers don't expect updates to happen synchronously and usually won't depend on that. Asynchronous data flows and state updates are so common in the React ecosystem that they probably won't irritate developers.

Implementation

One question regarding the implementation is, whether fleux should start returning promises or use the new async/await stuff or neither.

Should there be a way to somehow force synchronicity? I lean towards no.

As a consequence of this change, some tests will need to be updated that check against state updates.