Fix for escape fields when they are created in tsbd.Point

[michal800106]

I receive panic error when I'm trying to write message with commas and internal quotas.

# curl -G 'http://localhost:8086/query' --data-urlencode "q=CREATE DATABASE mmm"
{"results":[{}]}

# curl -XPOST 'http://localhost:8086/write?db=mmm' --data-binary '@t.g'
curl: (52) Empty reply from server

# cat t.g
{
  "database" : "mmm",
  "retentionPolicy" : "",
  "points" :  [ {
    "measurement" : "http_status",
    "tags" : {
      "service" : "identity-service",
      "hostname" : "host1",
      "url" : "http://localhost:35774/"
    },
    "timestamp" : "2015-06-16T09:28:26.000Z",
    "fields" : {
      "value_meta" : "{Hello\"{,}\" World}",
      "value" : 1.0
    }
  } ],
  "tags" : { }
}

The problem is that with creation tsbd.Point:

func NewPoint(name string, tags Tags, fields Fields, time time.Time) Point {
        return &point{
                key:    makeKey([]byte(name), tags),
                time:   time,
                fields: fields.MarshalBinary(),
        }
}

In function MarshalBinary is error. Because there are added quotas on begin and end of field.
Problem is that we receive string:

"{Hello"{,}" World}"

instead of expected by function scanFieldValue

"{Hello\"{\\,}\" World}"

The biggest problem occurs later because with wrong string function scanFieldValue splits string in comma and we receive:

panic: unsupported value type during encode fields: <nil>

goroutine 29 [running]:
github.com/influxdb/influxdb/tsdb.(*FieldCodec).EncodeFields(0xc2081084b0, 0xc208146e70, 0x0, 0x0, 0x0, 0x0, 0x0)

[otoolep]

@jwilder

[michal800106]

I rebased to your master and added new commit to fix what is escaped and in which order.
I create new pull request #3073