=str: #15802: TickSource cancellable

 - TickPublisher is now single Subscriber only, the TickSource is reusable (creates fresh publishers)
- also, fixing intermediate materialization for Source.concat
As I learned when working on the new IteratorPublisher with @viktorklang it is not this easy.
We should use the same approach as in IteratorPublisher (perhaps extracting common parts).
If you don't want to do it in this PR, we can leave a note in #16331