allow arbitrary client json attributes to be marged into keepalive results payload

    {
      "client": {
        "name": "example.com",
        "address": "127.0.0.1",
        "subscriptions": [ "a", "b" ],
        "keepalive": {
          "thresholds": {
            "warning": 60,
            "critical": 180
          },
          "handler": "keepalive",
          "attributes": {
            "refresh": 600
            "blah": "foo"
          }
        }
      }
    }