*: refactor storeInfo.

[disksing]

Fix #634.
Now the outputs are:

• Normal

{
  "store": {
    "id": 1,
    "address": "127.0.0.1:20160",
    "state": 0,
    "state_name": "Up"
  },
  "status": {
    "capacity": "229 GiB",
    "available": "80 GiB",
    "leader_count": 1,
    "region_count": 1,
    "start_ts": "2017-08-24T10:22:55+08:00",
    "last_heartbeat_ts": "2017-08-24T10:25:06.133247517+08:00",
    "uptime": "2m11.133247517s"
  }
}

• Down

{
  "store": {
    "id": 1,
    "address": "127.0.0.1:20160",
    "state": 0,
    "state_name": "Down"
  },
  "status": {
    "capacity": "229 GiB",
    "available": "80 GiB",
    "region_count": 1,
    "start_ts": "2017-08-24T10:22:55+08:00",
    "last_heartbeat_ts": "2017-08-24T10:25:16.133257177+08:00",
    "uptime": "2m21.133257177s"
  }
}

• Offline

{
  "store": {
    "id": 1,
    "address": "127.0.0.1:20160",
    "state": 1,
    "state_name": "Offline"
  },
  "status": {
    "capacity": "229 GiB",
    "available": "80 GiB",
    "region_count": 1,
    "start_ts": "2017-08-24T10:26:28+08:00",
    "last_heartbeat_ts": "2017-08-24T10:26:58.50635968+08:00",
    "uptime": "30.50635968s"
  }
}

• Down + Offline + Restart PD

{
  "store": {
    "id": 1,
    "address": "127.0.0.1:20160",
    "state": 1,
    "state_name": "Offline"
  },
  "status": {
    "region_count": 1
  }
}

[disksing]

Travis CI fails because etcd was too slow and pd was not able to bootstrap. I'm going to just restart it.

[huachaohuang]

Good, LGTM

[nolouch]

LGTM