Remove old scheduling objects from INBOX and oc_schedulingobjects via cron

[miaulalala]

When a scheduling event is in the past, the scheduling event should be removed from oc_schedulingobjects and the associated ICS in the scheduling inbox should be deleted: sabre-io/dav#1138

Any FREEBUSY request checks the schedulingobjects / a users inbox). The search will be limited by a start and end date. Since there is no dedicated timestamp on each entry of the table, but instead the query is done on a text column, this is where I would expect the largest performance impact.

[ChristophWurst]

Query analysis of a production instance. Time span is one week.

SELECT
  `uri`,
  `calendardata`,
  `lastmodified`,
  `etag`,
  `size`
FROM
  `oc_schedulingobjects`
WHERE
  `principaluri` = ?