If the user "opts in" (as in the parent commit), we can optimize
multiple INSERT statements to use table-level locking and undo logging.
There will be a change of behavior:

    CREATE TABLE t(a PRIMARY KEY) ENGINE=InnoDB;
    SET foreign_key_checks=0, unique_checks=0;
    BEGIN; INSERT INTO t SET a=1; INSERT INTO t SET a=1; COMMIT;

will end up with an empty table, because in case of an error,
the entire transaction will be rolled back, instead of rolling
back the failing statement. Previously, the second INSERT statement
would have been logged row by row, and only that second statement
would have been rolled back, leaving the first INSERT intact.