Fix DNSRPS code after struct dns_db refactoring

Fix DNSRPS code after struct dns_db refactoring

Commits ffa4757c792579e3bc8316df6ce4f47093e0bde3 and
77e7eac54c51c5e23bf6e0349237442ea105d02a inadvertently broke
DNSRPS-enabled builds:

  - the new member of struct dns_db that holds a reference count for the
    database is called 'references', not 'refcount',

  - a syntax error was introduced in the designated initializer for
    'rpsdb_rdataset_methods',

  - rpsdb_destroy() no longer takes a 'dbp' argument.

Address all of the above issues to make DNSRPS-enabled builds work
again.