Correct result type

[iv461]

Fixes the result type: It has two states, not three (I had managed to somehow confuse it with the promise state, now both concepts are separate).

Also makes the async/await API thread-safe:

• A race between promise resolution and rejection may occur when calling services since the timer task and the service response task may be scheduled into separate executor threads. They are now synchronized by an atomic exchange that says whether the promise was resolved/rejected yet. If the promise was already resolved/rejected, subsequent resolutions/rejections are ignored. This is lock-free and therefore the fastest way to prevent this race.
• TF callbacks are synchronized with a mutex. This makes it slower because a hashtable has to be copied now.