Support ephemeral instance lifecycle event tracking

[meatballhat]

This is not a full event audit per instance, but instead last-in captures the following named events with timestamps. The only event that is expected to occur multiple times is heartbeat.

• prelaunching - an SNS notification has been received with an autoscaling:EC2_INSTANCE_LAUNCHING transition for a given instance with token for confirmation after the instance "phones home" with...
• launching - an instance has "phoned home" once it begins to receive work. In the case of travis-worker, this happens when a "start hook" is executed.
• heartbeat - something has hit GET /heartbeats/{instance_id}, which is an unauthenticated route, so hopefully it was the instance. maybe.
• preterminating - an SNS notification has been received with an autoscaling:EC2_INSTANCE_TERMINATING transition for a given instance with a token for confirmation after the instance "phones home"
• terminating - an instance has "phoned home" once it has stopped receiving work. In the case of travis-worker, this happens when a "stop hook" is executed.

[codecov-io]

Current coverage is 59.84% (diff: 73.27%)

Merging #18 into master will increase coverage by 2.42%

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[meatballhat]

yey

[meatballhat]

a full lifecycle example:

{
  "data": [
    {
      "event": "prelaunching",
      "timestamp": "2016-09-18T21:40:31.192935087Z"
    },
    {
      "event": "launching",
      "timestamp": "2016-09-18T21:48:56.635388471Z"
    },
    {
      "event": "preterminating",
      "timestamp": "2016-09-18T22:56:22.333633842Z"
    },
    {
      "event": "heartbeat",
      "timestamp": "2016-09-18T22:56:22.508798533Z"
    },
    {
      "event": "terminating",
      "timestamp": "2016-09-18T22:56:22.604577654Z"
    }
  ],
  "meta": {
    "instance_id": "i-f9181dee"
  }
}