wait_any/wait_all are badly named

[stevenrbrandt]

The functions wait_any and wait_all are badly named, as they do not wait. Possibly when_all or when_any would be better names.

[hkaiser]

We already renamed wait_all to when_all and wait_any to when_any and re-added wait_all/wait_any with different semantics, e.g. actually waiting (see 76b4811).

This is still non-satisfactory, as when is now used with two different semantics: future<>::when attaches a continuation, while when_all/when_any are used to combine other asynchronous operations.

[hkaiser]

I think this can be closed as future::when has already been renamed to future::then.