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All databases in the codebase follow the same structure: a database is an associative container from DNS names to nodes, and each node is an associative container from RR types to RR data. Each database implementation (qpzone, qpcache, sdlz, builtin, dyndb) has its own corresponding node type (qpznode, qpcnode, etc). However, some code needs to work with nodes generically regardless of their specific type - for example, to acquire locks, manage references, or register/unregister slabs from the heap. Currently, these generic node operations are implemented as methods in the database vtable, which creates problematic coupling between database and node lifetimes. If a node outlives its parent database, the node destructor will destroy all RR data, and each RR data destructor will try to unregister from heaps by calling a virtual function from the database vtable. Since the database was already freed, this causes a crash. This commit breaks the coupling by standardizing the layout of all database nodes, adding a dedicated vtable for node operations, and moving node-specific methods from the database vtable to the node vtable.