Move some global variables to SQL_CATALOG

This task should be enhanced to list all global variables that makes sense to be made
catalog specific.

The current list is:

• query_cache status (on/off)
• Log_to_csv_event_handler
• opt_userstat_running
• event_scheduler

Like with STATUS VARIABLES, https://mariadb.com/kb/en/catalog-status-variables/, the GLOBAL and SESSION variables will be extended with CATALOG and SERVER variables.

When using catalogs you have:

• SERVER - Server-wide variables
• CATALOG & GLOBAL - Catalog specific variables (which some can be server-wide and not changeable by the tenant/catalog user)
• SESSION - Session specific variables.

When not using catalogs you have:

• SERVER, CATALOG and GLOBAL - Server-wide variables
• SESSION - Session specific variables.