=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
Yes, I understand that. But it's a nice tradeoff, sometimes small delay v.s. simpler code and no race.