Prevent spurious cron emails due to recent MySQL changes.

MySQL has recently added a warning to mysqldump output to alert users that Events are not dumped by default (see MySQL bug #55587). The warning looks like this: "-- Warning: Skipping the data of table mysql.event. Specify the --events option explicitly."

This warning causes spurious emails from cron. It doesn't hurt to back up Events, as far as I know, so we add the --events flag to avoid the warning.

Unfortunately this change causes permission errors when processing the performance_schema db, even though mysqldump should ignore that db. Hence, we explicitely remove it from the list of databases to back up.