[autotest] Record ending time for special tasks in afe_special_tasks

Add a new field time_finished in afe_special_tasks table. The new field records
when a special task is finished.
In SpecialTask.finish method, set the value of time_finished.

BUG=chromium:387918
DEPLOY=migrate,apache
TEST=run a verify job in local setup, confirm the new field has a value of the
verify task.

Change-Id: Ic570bc8ec1c594587c8ec6ee51c31ec4ad140a6a
Reviewed-on: https://chromium-review.googlesource.com/205291
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Fang Deng <fdeng@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
2 files changed