CompletableJob

[elizarov]

We have use-cases when we need an explicit Job object with children. However, the regular Job() constructor produces a job that cannot complete normally. You can only cancel() it, thus also cancelling all its children. Right now CompletableDeferred<Unit>() is used as such a primitive. It works final, but just looks quite unnatural, so the proposal is to add a dedicated CompletableJob() primitive that has a complete() method to complete its normally.