bucketed files: Switch to optimize the recursion.

* src/dctc/java/com/dataiku/dctc/file/BucketBasedFile.java: Here.
ls: Update the flags for the bucketed file recursions.

* src/dctc/java/com/dataiku/dctc/command/Ls.java: To enable recursion
  optimization.
style: Prefer isEmpty to length.

* src/dctc/java/com/dataiku/dctc/file/S3File.java: Faster, readable,
  easier.
s3: Optimize when we don't need to make a recursive list.

* src/dctc/java/com/dataiku/dctc/file/S3File.java: Use the previous
  defined flag.