Add repeatUntil() operator to remove kludge

Created by: lguychard

I came across this kludge while reading through code, and thought I'd try to find the Right Way ™️ to fix it because it seemed fun. The use case for the new repeatUntil() operator here seems legit to me: polling until a condition is met, while still mirroring values (vs. retryWhen() polling, which only emits in the absence of an error).

There may be an even simpler way to do this, in which case I'd love to be further enlightened. This is obviously fairly low-priority to review 🙂