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ARROW-15968: [C++] Update AsyncGenerator semantics to emit a terminal item only after all outstanding futures have completed

Unfortunately, this seems to have made the merge generator, which was already quite complicated, even more complicated.  I'd welcome any suggestions for simplification.  In the meantime, even though this one generator is more complicated, I think this allows us to simplify code using async generators considerably.

This is a prerequisite for #12468 because there is no way to keep the serial generator alive after the async generator has been destroyed (we can't use shared_ptr in this case)

Closes #12662 from westonpace/feature/ARROW-15968--only-emit-terminal-items-when-outstanding-tasks-finished

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>