tools/mariadb_query_io.py: new tool

[MaxKellermann]

I developed this script while analyzing a scalability problem in MariaDB. It turned about that MariaDB had a lot of contention on the global "dict_sys.latch" lock which caused all queries to become extremely slow.

This tool does not depend on userspace tracepoints, and can therefore be used on MariaDB without recompiling. I tested it on MariaDB 10.11.6 (Debian Bookworm).