The TransferFsImage class has grown several heads and is somewhat confusing to follow. This JIRA is to refactor it a little bit.
- the TransferFsImage class contains static methods to put/get image and edits files. It's used by checkpointing nodes. [the same static methods it has today]
- some common code from call sites of TransferFsImage are moved into TransferFsImage itself, so it presents a cleaner interface to checkpointers
- the non-static parts of TransferFsImage are moved to an inner class of GetImageServlet called GetImageParams, since they were only responsible for parameter parsing/validation.