Consider changing next() in ThingDiscovery

[zolkis]

In the TAG review of WoT Architecture there were couple of comments on Scripting API as well.

This issue tracks the comment that the next() method in ThingDiscovery resembles an async iterator: either use iterators or change the name?

Option A: keep using next() and add an editor's note that while this resembles async iterators, it is not actually based on them, due to constrained implementations.

Option B: change the name for next() to avoid confusions.

Option C: actually use async iterators.

[zolkis]

On the Scripting call on 22. July 2019 we agreed on Option A from above, i.e. to keep the next() method and done property at ThingDiscovery without making it an Iterable for time being, until understanding the impact on constrained implementations.