*: add trend api.

[disksing]

It's for visualization of cluster's activities.

Usage example:

$ curl -s "http://127.0.0.1:2379/pd/api/v1/trend?from=1513086582"  // unix epoch.
{
  "stores": [
    {
      "id": 1,
      "address": "127.0.0.1:20161",
      "state_name": "Up",  // Up/Down/Offline/Disconnect
      "capacity": 5368709120,  // in bytes.
      "available": 5279425783, // in bytes.
      "region_count": 64,
      "leader_count": 20,
      "start_ts": "2017-12-19T15:18:20+08:00",
      "last_heartbeat_ts": "2017-12-19T15:20:00.874237675+08:00",
      "uptime": "1m40.874237675s",
      "hot_write_flow": 89884,   // sum of all hot regions's write flows, in bytes.
      "hot_write_region_flows": [  // a list of region's write flows, in bytes.
        89884
      ],
      "hot_read_flow": 0,
      "hot_read_region_flows": null
    },
    {
      ...
    },
   
  ],
  "history": {
    "start": 1513086582,  // start time of the history.
    "end": 1513086616,    // end time of the history.
    "entries": [
      {
        "from": 4,    // source store id.
        "to": 1033,  // target store id.
        "kind": "region",  // resource kind, "region" or "leader".
        "count": 9   // total count of resource during the time.
      },
      {
        "from": 5,
        "to": 1033,
        "kind": "leader",
        "count": 8
      },
      {
        ...
      }
    ]
  }
}

[disksing]

PTAL @Connor1996 @siddontang @nolouch

[Connor1996]

LGTM

[c4pt0r]

I think there is no order guarantee.

[disksing]

Yes, in fact we do not care about the order, only the changing trend.

[tiancaiamao]

Not thread-safe here by design?

[disksing]

It's safe as each request are accessing different copy.

[tiancaiamao]

histories := make( []schedule.OperatorHistory, 0, c.histories.Len())

Len() is O(1)

[tiancaiamao]

LGTM