"getTopNSlowService" service, change the attribute name from TPS to callsPerSec.

[peng-yongsheng]

1. Changed the attribute name from TPS to callsPerSec.
2. Calculate the callsPerSec.

INPUT

{
  getTopNSlowService(duration: {start: "2017-06-01 1002", end: "2017-06-01 1005", step: MINUTE}, topN: 10) {
    id, name, avgResponseTime, callsPerSec
  }
}

OUTPUT

{
  "data": {
    "getTopNSlowService": [
      {
        "id": "-2",
        "name": "/dubbox-case/case/dubbox-rest",
        "avgResponseTime": 2000,
        "callsPerSec": 20
      },
      {
        "id": "2",
        "name": "org.skywaking.apm.testcase.dubbo.services.GreetService.doBusiness()",
        "avgResponseTime": 1000,
        "callsPerSec": 20
      }
    ]
  }
}

[coveralls]

Coverage decreased (-0.01%) to 19.452% when pulling 2caa29a on peng-yongsheng:feature/getTopNSlowService into e90cfc8 on apache:master.