Allow innodb_log_file_size to change without server restart

Currently, if the parameters of the redo log change, InnoDB in the MariaDB Community Server will rebuild the redo log at server startup.

MariaDB Enterprise Server 10.5 and 10.6 allow dynamic tuning of the redo log parameters, rebuilding the redo log in a crash-safe manner without restarting the server.

SET GLOBAL innodb_log_file_size=…;

Before MDEV-14425 changed the redo log file format in 10.8, we were be unable to enable or disable innodb_encrypt_log without server restart, because starting with MDEV-12041 in MariaDB 10.4, the encrypted redo log blocks will have 4 bytes less payload per 512-byte log block.

Log resizing is tied to checkpoints. We can start writing a second redo log ib_logfile101 with the requested new size, starting from something close to the last written log sequence number. On log checkpoint completion, we can switch files, provided that the checkpoint LSN was not earlier than the start LSN of the resized log file. When resizing to a small log file during a heavy write workload, multiple checkpoints may be necessary.

While technically it would be possible to rebuild the log for changing innodb_encrypt_log, this task does not implement it, because it would require a non-trivial transformation between the log record streams that are being written to the current log file (ib_logfile0) and the future log file (ib_logfile101 that will replace ib_logfile0).

Rebuilding the log file will obviously cause disruption to mariadb-backup --backup, because the old log file will stop receiving writes once the server has switched to another log file. This could be addressed in MDEV-14992 by letting the server provide a log record stream directly.