Restoring kite-data-flume to kite-data modules.

This was accidentally disabled in 24ed582d.
CDK-304: Backporting DatasetSink from flume.

This adds the kite-data-flume-sink module. This is for
backward-compatibility and to make the sink available until it is
released in upstream flume.
CDK-304: Backporting FLUME-2320 for DatasetSink.

FLUME-2320: Fix deadlock by removing writer lock.
CDK-304: Add DatasetSink to release notes.

This also explains why we don't currently recommend it or use it in the
examples.