chg: dev: Add a refcount to the vecheaders

This MR changes the way the ownership of the vecheaders is tracked. Before this MR, the ownership of the vecheader was implicitely tracked through a mix of the refcount on the node owning the header, the external refcount of the same node and the version. This has some adverse consequences in terms of contention, such as that querying A and AAAA glue hits the same refcount.

This MR adds a refcount to the vecheader itself, allowing it to exist independently of the node it is contained in. On its own, this would create a cycle, where the node has a reference to the header, which has a reference to the heap, which in turn has a reference to the node.

To break this cycle, this MR also moves from an "intrusive" heap, to a more traditional one where pointers to the node and vecheader in the heap are stored in a hashmap.

Merge branch 'alessio/vecheader-refs' into 'main'

See merge request isc-projects/bind9!11397