Fix DNS_ZONEFLG_LOADPENDING handling

Michał Kępień requested to merge fix-loadpending-handling into master

See RT#47076.

This MR prevents memory leaks caused by improper handling of DNS_ZONEFLG_LOADPENDING coupled with overlapping rndc reload invocations (i.e. issuing another rndc reload before the previous one is completed).

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