* Added "started_at" and "finished_at" to task executions' printout.
* Added "duration" to task executions that's calculated as a delta
between "finished_at" and "started_at".
* Removed "updated_at" from task executions printout since it doesn't
carry any relevant info for a user, "finished_at" makes more sense.
Change-Id: Ie0bd01135548bc9fb5137368a1933d7b8b3c11e5