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ARROW-14009: [C++] Seed parallellism in SourceNode

Previously we synchronously called InputReceived in the loop that consumed the source generator. This meant that we only ever processed one batch at a time (barring a pipeline breaker). Instead, submit batches to an executor and use an AsyncTaskGroup to track completion.

Closes #11165 from lidavidm/arrow-14009

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