mysql plugin: Subquery cache

The subquery cache was added in mysql-5.6 and mariadb-5.3.2.

Mariadb provides separate status variables to determine if enabling/disabling the feature will provide benefits.

As such this plots it with the rest of the query cache numbers.

https://mariadb.com/kb/en/subquery-cache/
https://dev.mysql.com/doc/refman/5.6/en/query-cache-operation.html