The event scheduler has a THD which is used for e.g. keeping track
of the timing of the events. Thus, each scheduling of an event will
make use of this THD, which in turn allocates memory in the THD's
mem root. However, the mem root was never cleared, and hence, the
memory occupied would monotonically increase throughout the life
time of the server.

The root cause was found by Jon Olav Hauglid, and this fix clears the
THD's mem root for each event being scheduled.

Change-Id: I462d2b9fd9658c9f33ab5080f7cd0e0ea28382df