Audit log inside InnoDB

As noted in MDEV-38126, the audit plugin could work with significantly smaller overhead if there was an option to redirect the output to the InnoDB redo log, similar to the binlog_storage_engine=innodb option (MDEV-34705, MDEV-38304). In this way, we would leverage the efficient buffer allocation MDEV-21923 and allow concurrent writes of several audit events to log_sys.buf with each writer holding a shared log_sys.latch. The current implementation can only write one message at time, under exclusive lock_operations.

This work would involve developing a tool that extracts audit log messages from the InnoDB redo log.