Check unnecessary usage of msg.Clone()

[msiebeneicher]

We need to check unnecessary usage of msg.Clone().

Here an example which make no sense with the new clone handling from v0.5.x:

source producer/scribe.go:203:

// Convert messages to scribe log format
	for idx, msg := range messages {
		currentMsg := msg.Clone()

		category, exists := prod.category[currentMsg.GetStreamID()]
		if !exists {
			if category, exists = prod.category[core.WildcardStreamID]; !exists {
				category = core.StreamRegistry.GetStreamName(currentMsg.GetStreamID())
			}
			metricName := scribeMetricMessages + category
			tgo.Metric.New(metricName)
			tgo.Metric.NewRate(metricName, scribeMetricMessagesSec+category, time.Second, 10, 3, true)
			prod.category[currentMsg.GetStreamID()] = category
		}

		logBuffer[idx] = &scribe.LogEntry{
			Category: category,
			Message:  string(currentMsg.GetPayload()),
		}

		tgo.Metric.Inc(scribeMetricMessages + category)
	}

The file producer had the same issue which i'll fix in #173 by providing a new message method GetOrigStreamID. The main reason for message cloning was often to get the original stream id, which is not necessary anymore and only need resources.

// GetOrigStreamID returns the original/first streamID
func (msg *Message) GetOrigStreamID() MessageStreamID {
	return msg.orig.streamID
}

[msiebeneicher]

Added method by 0f71ba3