[C++] Potential memory leak at shutdown if an exec plan with a scanner fails or is aborted immediately before shutdown

I'm primarily creating this so we can remember to make a test for this. This problem should be solved as part of ARROW-16072. When the scanner fails it simply discards references to the various scanner AsyncGenerators. However, some I/O tasks may still have references to these generators and so some parts of the scanner survive after the plan itself is marked complete. If there is an immediate shutdown then these parts will not be properly disposed of even though the plan is marked complete and it will show up as a memory leak.

Example:

https://pipelines.actions.githubusercontent.com/serviceHosts/8bb0d999-3387-4c48-9fa6-c66c718a46e2/_apis/pipelines/1/runs/359690/signedlogcontent/4?urlExpires=2022-07-25T14%3A43%3A01.2797488Z&urlSigningMethod=HMACV1&urlSignature=GS3lS09Q9sTRweN%2B8UEu2GwUGc%2FbO9eyH27FRKumbrg%3D

**Reporter**: [Weston Pace](https://issues.apache.org/jira/browse/ARROW-17198) / @westonpace

_{**Note**: *This issue was originally created as [ARROW-17198](https://issues.apache.org/jira/browse/ARROW-17198). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

No source file or test path is identified. Start by reviewing ARROW-16072 and the scanner/exec-plan shutdown behavior it covers; done means adding a regression test for a failed or aborted scanner immediately before shutdown and verifying that no memory leak remains.

Written by the indexing model from the issue text.