[9.20] fix: usr: Fix zone deletion issue

A secondary zone could initiate a new zone transfer from the
primary server after it had been already deleted from the
secondary server, and before the internal garbage collection
was activated to clean it up completely. This has been fixed.

Closes #5291

Backport of MR !10449

Merge branch 'backport-5291-zone-delete-bug-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10496