Fix InnoDB critical bug #7496; we scan the InnoDB data dictionary also at a normal mysqld startup, and create the spaces, so that we know the mapping space id -> .ibd file name; fix an infinite loop if DISCARD TABLESPACE coincides with INSERT or some other table operation; fix a potential crash if DISCARD TABLESPACE coincides with a cascaded FOREIGN KEY operation in the same table; do not allow DISCARD TABLESPACE of a referenced table if FOREIGN_KEY_CHECKS=1