chg: dev: Extract the resigning heap into a separate struct

In the current implementation, the resigning heap is part of the zone
database. This leads to a cycle, as the database has a reference to its
nodes, but each node needs a reference to the database.

This MR splits the resigning heap into its own separate struct, in order
to help breaking the cycle.

Merge branch 'alessio/split-qpzone-heap-from-qpdb' into 'main'

See merge request isc-projects/bind9!10706