chg: dev: Mark the related slabheader as visited on cache hit

A cache hit only marked the looked-up header as SIEVE-visited, leaving
its related header (the flattened counterpart) a candidate for eviction.
Mark both so related slabheaders age together.

Merge branch 'ondrej/mark-related-header-visited-on-cache-hit' into 'main'

See merge request isc-projects/bind9!12306