Calling KinesisProducerActor.props already starts a KinesisProducer

[aquamatthias]

While playing with the KinesisSinkGraphStage I realized, that a kinesis producer is started even before the actor has been created, since a call to props starts the producer behind the scenes. It was surprising for me and I think it could be surprising for others as well.

I see 2 scenarios which probably should be supported:

• an already created KinesisProducer should be used
• a new KinesisProducer should be created on demand, if the actor starts