While the record type MLOG_UNDO_INIT feels redundant, it does save
redo log volume, and subtle changes to redo log volume appear to
make a big difference for the log_sys.mutex contention.

trx_undo_parse_page_init(): Allow type==0, which is what we write
since MDEV-12288. Parse type in a simpler way; it always was written
as a single byte.

trx_undo_page_init(): Write a MLOG_UNDO_INIT record.