Need `Completable.asWorker()` solution when `Worker.createSideEffect()` is removed.

[steve-the-edwards]

Completable, by definition is emitting Nothing. We can't handle that as a Worker and that is not needed because of runningSideEffect.

See #262

[steve-the-edwards]

The solution is just not to use a Worker for Completable.

if you have

myCompletable: Completable
// use
context.runningSideEffect {
  myCompletable.await()
}

[steve-the-edwards]

Closed by #1237