[9.18] fix: chg: Improve performance when looking for the closest encloser when retur...

[9.18] fix: chg: Improve performance when looking for the closest encloser when returning NSEC3 proofs

Use the fact that the database returns the longest matching part of the requested name to find the required NSEC3 record. If there are multiple versions present in the database we may have to search further.

Closes #4460

Backport of MR !9436

Merge branch 'backport-4460-auth-nsec3-many-labels-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9439