Remove db_nodelock_t in favor of reference counted qpdb

Remove db_nodelock_t in favor of reference counted qpdb

This removes the db_nodelock_t structure and changes the node_locks
array to be composed only of isc_rwlock_t pointers.  The .reference
member has been moved to qpdb->references in addition to
common.references that's external to dns_db API users.  The .exiting
members has been completely removed as it has no use when the reference
counting is used correctly.

(cherry picked from commit 431513d8b3cc595a7612479fd7811e959501e6dd)