Since high-level index tables do not participate in thr_multi_lock(), added
explicit call to THR_LOCK::start_trans(). This is needed mostly for Aria to
handle transaction logging.
* 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
