Fix hang when PATCHing zone during rectify

[pieterlexis]

Before, we would spawn a new UeberBackend in the DNSSECKeeper, but there
was already a transaction going on, so the rectify would never finish,
as rectifyZone would not return.

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• read the CONTRIBUTING.md document
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• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)