chg: dev: Add a circular reference between slabtops for type and RRSIG(type)

Previously, the slabtops for "type" and its signature was only loosely
coupled and the headers could expire at different time (both TTL and LRU
based expiry).  Add a .related member to the slabtop that allows us to
expire the headers in both related headers and also optimize the lookups
because now both slabtops are looked up at the same time.

Closes #3396

Merge branch '3396-bind-rrsigs-to-records' into 'main'

See merge request isc-projects/bind9!10985