While the redo log format was changed in MariaDB 10.3.2 and 10.3.3
due to MDEV-12288 and MDEV-11369, it should be technically possible
to upgrade from a crashed MariaDB 10.2 instance.

On a related note, it should be possible for Mariabackup 10.3
to create a backup from a running MariaDB Server 10.2.

mlog_id_t: Put back the 10.2 specific redo log record types
MLOG_UNDO_INSERT, MLOG_UNDO_ERASE_END, MLOG_UNDO_INIT,
MLOG_UNDO_HDR_REUSE.

trx_undo_parse_add_undo_rec(): Parse or apply MLOG_UNDO_INSERT.

trx_undo_erase_page_end(): Apply MLOG_UNDO_ERASE_END.

trx_undo_parse_page_init(): Parse or apply MLOG_UNDO_INIT.

trx_undo_parse_page_header_reuse(): Parse or apply MLOG_UNDO_HDR_REUSE.

recv_log_recover_10_2(): Remove. Always parse the redo log from 10.2.

recv_find_max_checkpoint(), recv_recovery_from_checkpoint_start():
Always parse the redo log from MariaDB 10.2.

recv_parse_or_apply_log_rec_body(): Parse or apply
MLOG_UNDO_INSERT, MLOG_UNDO_ERASE_END, MLOG_UNDO_INIT.

srv_prepare_to_delete_redo_log_files(),
innobase_start_or_create_for_mysql(): Upgrade from a previous (supported)
redo log format.