Commits ffa4757c79 and
77e7eac54c 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.