=str #15355 Fix race in Tee causing a RequestMore overtaking ExposedPublisher

This also makes the secondary subscription lazy, which leaves more wiggling room for #15086. It might be possible to restore the eager behavior, but currently I see less complications with this approach.

Fixes #15355
The primary driving force was not the simplification though, as it was pointed out in the ticket, since this pays with a small delay in certain cases.