Rename Segment to Task in Flight (#24753)

In Fizz this got split into Task and Segment. We don't have a concept of
Segment in Flight yet because we don't inline multiple segments into one
"Row". We just emit one "Row" for each Segment if something suspends.

This makes Flight non-deterministic atm but that's something we'll want to
address.

Regardless, right now, this is more like a Task than a Segment.