Fix DNSRPS code after struct dns_db refactoring

Commits ffa4757c and 77e7eac5 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.

We should preferably merge this before the March code freeze as the breakage was introduced in this release cycle.

See #3882 (closed)/!7558 (merged)

See also !7601 (merged) for an attempt at detecting the type of breakage addressed by this MR in advance.

Closes #3891 (closed)