fix timestamps in avro_record.go
fix timestamps in avro_record.go

MetricsMessage.Timestamp should be time.Now().Unix() (int64), and the
timestamps for datapoints should all be formatted to time.RFC3339 and in
UTC, not the system's timezone.

dcos/dcos-metrics,dcos/dcos-metrics,dcos/dcos-metrics,dcos/dcos-metrics,dcos/dcos-metrics