api: add StartedAt in Node.DrainStrategy

[lgfa29]

The new StartedAt field records the time the current drain process started in a node.

Sample response:

  "DrainStrategy": {
    "Deadline": 3600000000000,
    "ForceDeadline": "2019-11-13T23:49:32.696485Z",
    "IgnoreSystemJobs": false,
    "StartedAt": "2019-11-13T22:49:08.162419Z"
  }

Fixes #6694

[jrasell]

Would it be preferable to setup a var of time.Time outside the two if statements and then assign this within the first if statement as needed to also be used in the second? This would mean the started time and deadline are consistent with the user passed DrainStrategy.Deadline.

[drewbailey]

I like @jrasell's idea of having a var startedAt time that we could then use here as well to add on to. There seems to be at least one more location calling time.Now that could also benefit from using a shared variable

[lgfa29]

Ah! That makes perfect sense, thanks for the feedback @jrasell @drewbailey

[lgfa29]

@jrasell @drewbailey I refactored the logic to use a single value for current time. Let me know if it's better.

I also remove the check for node.DrainStrategy.StartedAt.IsZero() because it's not relevant. I initially had it as args.DrainStrategy.StartedAt.IsZero() for backwards compatibility, but I don't think it makes sense to check it against node.

[drewbailey]

LGTM!

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