MDEV-15418 changed InnoDB to use the READ UNCOMMITTED isolation level
when the transaction recovery is disabled by setting
innodb_force_recovery to 5 or 6. Alas, CHECK TABLE would still
internally use REPEATABLE READ. If the server was started without
purge running into completion (and resetting all DB_TRX_ID thanks
to MDEV-12288), this would cause errors about any DB_TRX_ID that
had not been reset to 0.