Use batch event submission

[ePaul]

Currently, the event submitter does a separate HTTP request for each single event in the queue.
When submitting many events (e.g. for snapshot creation), this has some overhead (network roundtrips, HTTP headers, ...) and slows down the submission.

Instead, we should use the batching-feature of Nakadi (and Fahrschein) to submit several events for the same event type in a batch. Care must be taken to delete only the successful ones from the eventlog table.

[BGehrels]

The current Nakadi Producer API does not define a Batch size limit but it "is planned to be released this quarter" (April 21, 2017). So i guess we should put this ticket on hold until the limit is defined.

[ePaul]

According to the Nakadi project, they don't have a fixed limit. Batches of up to 50 KB are included in the SLO measuring (of Zalando's internal Nakadi-as-a-service), so that might be a useful guideline.

[BGehrels]

Current docs say:

What is the maximum batch size Nakadi could send out? Nakadi maximum batch size is 50 MB of uncompressed data.

[BGehrels]

Fixed with #113 and experiences are good so far