MDL-66253 calendar: New indexes to reduce full table scans

MDL-66253 calendar: New indexes to reduce full table scans

The eventtype index will stop full table scans when the Event API
retrieves events for a group or category and there are a large number
of groups or categories so it cannot use the groupid or categoryid
indexes efficiently.

The modulename-instance index will improve the performance of queries
used by activity backup, deletion and visibility changes where all
the events for an individual activity need to be found.