chg: dev: Remove locking from rdataslab_getownercase()

Under normal circumstances, the case bitfield in the slabheader should
be set only once.  By actually (soft-)enforcing this, the read locking
can be completely removed from the rdataslab_getownercase() as we can
check whether the case has been already set or not and making everything
immutable once the case has been set.

Merge branch 'ondrej/remove-locking-from-slabheader-ownercase' into 'main'

See merge request isc-projects/bind9!10843