Add support for originals in message

[arnecls]

The intention of this task is to ease development for plugin writers regarding spooling support and adds more consistency to the way messages are modulated and forwarded.
By removing the support for message modification from the routers we can define an "original" to be generated by the consumer and can be sure no additional modification has been made before the message reaches a producer. By dropping only original messages we avoid double-formatting of messages e.g. sent to a spooling producer.

• The message generated from by a consumer AFTER applying modulators is seen as "original"
• A function "CloneOriginal" has to be implemented in the Message struct
• Remove formatter support from routers (see Rename "stream" plugins to "router" #80)
• When dropping a message a clone of the original is ALWAYS dropped (with streams set correctly)
• Modulating of messages has to move out of the concrete producer implementation unless SimpleProducer is used as a base struct (i.e. as of the current state BufferedProducer calls Modulate)
• Storing / generating originals has to be removed from all producers doing such a thing

[arnecls]

@msiebeneicher from our discussion:

type Message struct {
	data		MessageData
	orig		MessageData
	prevStreamID	MessageStreamID // history
	source		MessageSource   // back channel	  <- optional
	timestamp	time.Time	// time of creation  
	sequence	uint64		// weak ordering     <- raus?
}

type MessageData struct {
	key		[]byte		// later
	payload		[]byte
	streamID	MessageStreamID
}