Have Audit Plugin rotate logs at FLUSH LOGS

Currently Audit Plugin ignores FLUSH LOGS.

Utilities like logrotate rely on commands like mysqladmin flush-logs (converted to sql FLUSH LOGS).

MariaDB Audit Plugin as subsystem generating a log should behave consistently and react to FLUSH LOGS.

Probably we should add a category(AUDIT LOGS?) for it as specified in the manual of FLUSH LOGS for selective flushing:

https://mariadb.com/kb/en/mariadb/flush/