Reduce redo log volume for undo tablespace initialization

The function trx_rseg_header_create() is emitting a large amount of low-level redo log records for initializing an array of data to FIL_NULL (0xff bytes). In MDEV-13564, the mini-transaction log size for initializing an undo tablespace exceeds 1 megabyte, triggering some warnings and requiring a slightly bigger buffer pool size to avoid a hang during crash recovery.

We should introduce higher-level redo log record types for initializing undo log tablespaces. Ultimately, related to MDEV-14425 we should introduce a redo log format that only encodes a page number at most once per mini-transaction, instead of encoding it for every single operation. The introduction of redo log record types will require updating the redo log format tag, and thus prevent a crash-downgrade to earlier versions.