Before this change, `promises.timesLimit()` created `concurrency - 1`
too many promises. The only users of this function use a concurrency
of 500, so this meant that 499 extra promises were created each time
it was used. The bug didn't affect correctness, but it did result in a
large number of unnecessary database operations whenever a pad was
deleted. This change fixes that bug.
Also:
* Convert the function to async and have it resolve after all of the
created promises are resolved.
* Reject concurrency of 0 (unless total is 0).
* Document the function.
* Add tests.
John McLear