* mhnsw:
  * use primary key, innodb loves and (and the index cannot have dupes anyway)
    * MyISAM is ok with that, performance-wise
  * must be ha_rnd_init(0) because we aren't going to scan
    * MyISAM resets the position on ha_rnd_init(0) so query it before
    * oh, and use the correct handler, just in case
  * HA_ERR_RECORD_IS_THE_SAME is no error
* innodb:
  * return ref_length on create
  * don't assume table->pos_in_table_list is set
  * ok, assume away, but only for system versioned tables
* set alter_info on create (InnoDB needs to check for FKs)
* pair external_lock/external_unlock correctly