[9.20] chg: dev: Refactor reference counting in both QPDB and RBTDB

Clean up the pattern in the newref() and decref() functions in QP and RBTDB databases.  Replace the `db_nodelock_t` structure with plain reference counting for every active database node in QPDB.

Related to #5134

Backport of MR !10006

Merge branch 'backport-5134-refactor-decref-functions-in-qpdb-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10035