[chippison]

DEV-20866

Follow-up to matomo-org/matomo#25549, which makes the tracker connection refuse every query and the commit after an error that rolled back the transaction.

If a tracking step caught such an error on the last query of a request set, nothing noticed until the lock or commit queries, which then failed outside the retry path. The worker crashed, and the unlock in the error handler failed too, so the lock stayed held until it expired.

• After each tracked request the handler checks whether the transaction was rolled back, so the set goes down the existing retry path.
• When processing fails, the processor rolls back before it releases the lock.
• The retry check uses core's TransactionErrors when it exists.

Both checks are skipped on Matomo versions without the core change. The new HandlerTest case needs the core PR, and is skipped without it.

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