Added support of `max_completion_time` in docker executor.
Added support of `max_completion_time` in docker executor.

If `TaskInfo.max_completion_time` is set, docker executor will kill
the task immediately. We reuse the `shutdown` method to achieve a
forced kill ignoring any `KillPolicy`.

Framework should only received a `TASK_FAILED` state with
`REASON_MAX_COMPLETION_TIME_REACHED` reason.

Review: https://reviews.apache.org/r/66283/

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