Gradually scale the submission speed so Nakadi can catch up

[ePaul]

Nakadi has problems when suddenly much more events are sent than previously, as its autoscaling takes some time.

To not overload Nakadi (and thus negatively influcence other event types), when submitting bigger amounts of events, we should gradually increase the throughput instead of trying to submit all at once.

Related: #48 (use batch submission).