fix: dev: Acquire the database reference before possibly last node release

Acquire the database reference in the detachnode() to prevent the last
reference to be release while the NODE_LOCK being locked.  The NODE_LOCK
is locked/unlocked inside the RCU critical section, thus it is most
probably this should not pose a problem as the database uses call_rcu
memory reclamation, but this it is still safer to acquire the reference
before releasing the node.

Closes #5194

Merge branch '5194-fix-assertion-failure-while-reference-counting-qpdb' into 'main'

See merge request isc-projects/bind9!10155