[C++] Simplify async scheduler by removing the need to call End

[asfimport]

Right now the AsyncScheduler requires that we call End. This is to distinguish between a scheduler running out of tasks because it hit an idle period (in which case it shouldn't mark itself finished) and a scheduler running out of tasks because it is truly done (in which case it should mark itself finished).

It should be possible to require that scheduler tasks only be added by existing tasks. When creating a scheduler one would supply an "initial task" to bootstrap the first set of tasks. In this case we could get rid of the End method because the only time a scheduler will run out of tasks is when it is finished.

Reporter: Weston Pace / @westonpace
Assignee: Weston Pace / @westonpace

PRs and other links:

• GitHub Pull Request #14524

_{Note: This issue was originally created as ARROW-17509. Please see the migration documentation for further details.}

[asfimport]

Weston Pace / @westonpace:
Issue resolved by pull request 14524
#14524