Fix map()

[schmidt-sebastian]

Turns out that using foreach() for map() doesn't quite work. These two functions do very different things:

• Map takes the result of persistence promises and converts them to an array.
• forEach takes a list of values and "converts" them to a PersistencePromise by passing them through a callback.

map is so far only used in a single test that doesn't look at the return values, and hence nothing broke. I changed that test to use waitFor.

Alternatively, we could also remove map, but it might come in handy and is not all that trivial to implement (as I have just shown).