Automatically upgrade session isolation transaction levels to READ-COMMITTED for MySQL

In 3.5 we enforced a global MySQL isolation level of READ-COMMITTED. CONF-22071 will both allow for user customisation of their session isolation levels to READ-COMMITTED, and set the session isolation level to READ-COMMITTED for new Confluence instances. We should probably upgrade old MySQL instances to use READ-COMMITTED on a session level as well without users having to customise.