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ARROW-13417: [C++] The merged generator can sometimes pull from source sync-reentrant

A couple of things were at play here.

First, the parallel CSV reader added a cleanup step to the async vector generator which made it no longer sync-reentrant safe.  This served as a useful canary to detect this issue.

Second, there was a test in place to ensure that the merged generator did not pull from the individual subscriptions in a sync-reentrant manner.  However, this test was not guarding the source generator.

Finally, the bug itself was pretty straightforward.  The source could be pulled whenever a subscription finished and so if two subscriptions finished at the same time it would cause a race condition (or if a subscription finished very quickly and the initial pull was still adding subscriptions).

Closes #10758 from westonpace/ARROW-13417--c-the-merged-generator-can-sometimes-pull-from

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