refactor HDFS state and it's tests

Refactor HDFS state and it's tests. Add CheckpointState as the super trait of it.

This first refactoring attempt is not changing any behavior of the origin HDFSState. The intention is to separate out the super trait, CheckpointState.

Also refactored the tests to remove code duplication.

Why is this no longer private? it seems like it should always be such