MariaRocks: rocksdb.rocksdb fails with a duplicate key error
MariaRocks: rocksdb.rocksdb fails with a duplicate key error

In MariaDB, Field::make_sort_key stores NULL-indicator byte for
the field.
In MySQL, it doesn't, so MyRocks stores the NULL-indicator itself.

Switch to using Field::sort_string, which is the same as
Field::make_sort_key in MySQL.

natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2