tools/mariadb_query_io.py: new tool

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).

Author: MaxKellermann