MariaDB 10.2 refuses to start up after clean shutdown of MariaDB 10.3

MariaDB 10.3.2 changed the InnoDB redo log format tag.
By design, this causes older MariaDB versions to refuse downgrade on InnoDB files that had been modified with MariaDB 10.3.
We should allow MariaDB 10.2 to downgrade the 10.3 redo log if it is logically empty. We must keep refusing crash-downgrade, because 10.2 lacks the code to deal with some 10.3 redo log record types (and some undo log record types and B-tree format changes as well).