feat: Cancel running backfills on reaching failure threshold

[tomasfarias]

Problem

Besides pausing batch exports when they exceed a failure threshold, they should also cancel any ongoing backfills.

Changes

• Cancel ongoing backfills when finishing a batch export if reached failure threshold.

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Does this work well for both Cloud and self-hosted?

How did you test this code?

Added a unit test.

[posthog-bot]

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