Use generic Dataset type in DatasetRepository

While working on dataset URIs, CDK-385, I noticed that we can use generics to return concrete Dataset types instead of additional DatasetRepository types, like RandomAccessDatasetRepository. For example, rather than requiring two methods, load and loadRandomAccess, we can use one parameterized by the type that is expected:

// using this method definition . . .
public class Datsets {
  public <E, D extends Dataset<E>> D load(URI uri) { ... }
}

// this will work with RandomAccessDataset
RandomAccessDataset<MyRecord> = Datasets.load("dataset:hbase:zk1/records");

We should update the DatasetRepository interface so it can be used for HBase and get deprecate RandomAccessDatasetRepository.

I suggest we do this for 0.15.0 so the new dataset URI code can be used with HBase without adding a loadRandomAccess method to be removed later.