CDK-863: Deprecate DatasetWriter flush/sync, add Flushable.

This deprecates the DatasetWriter#flush and #sync methods, which are
replaced by Flushable#flush. Sync needed to call flush to guarantee the
records were durable, but flush was still exposed, requiring callers to
flush and sync, with sync calling flush immediately afterwards. Instead,
the new Flushable interface carries a stronger guarantee that records
have not only been flushed to the underlying buffer, but that they are
persisted to disk also.