Add 9.3 worker_checksums_verify_file support

Use SPI to query pg_class where relmapper support isn't available to give
relation Oid based on (tablespace, relfilenode).

There is still a TODO item concerning mapped relations, where we cannot
reasonably detect a relation's Oid based on the given tablespace relfilenode
combination. So for now, 9.3 support cannot be considered complete.