refactor HDFS state and it's tests. Add CheckpointState as the super trait of it
fix deleteOnExit
make sure to delete temp folder even when test fails
remove mix-in, use composition for CheckpointStore and CheckpointState
remove unnecessaru braces, reformat the comments to be less than 100 chars
reformat
fix tmp dir
Merge pull request #532 from tsdeng/refactor_hdfs_state

refactor HDFS state and it's tests