chg: dev: Reference count and flatten the cache slabheader storage

Internal refactoring of the cache database (qpcache) with no functional
change. The slab headers that hold cached rdatasets are now reference
counted and own their memory context and node reference directly, so a
header can outlive the cleaning of its node and be reclaimed
independently of it. Building on that, the per-type slabtop container is
folded into the slab header itself, removing a level of indirection and
one allocation per cached type.

Merge branch 'ondrej/slabheader-reference-counting' into 'main'

See merge request isc-projects/bind9!12285