Fix #304: Expose `active` property, add automatic cleanup

[magro]

This PR addresses issue #304:

• Expose OutboxProcessor.active property

  So that external scheduling of the cleanup could take that into account to prevent parallel cleanup jobs (according to the first option stated in issue #304).

• Add automatic cleanup of processed outbox records

  This is implemented in a backwards compatible manner, i.e. existing integrations would have to be adjusted to use automatic cleanup.

  The cleanup is done by the active instance holding the lock. It's using a separate executor/scheduler, so that outbox processing cannot be negatively affected / blocked by the cleanup.

mockito in the classic spring module had to be updated to be compatible with awaitility (prevent conflicts with different hamcrest versions).

[github-actions[bot]]

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