Sessions/legacy.sessions implementation

[glotchimo]

The legacy design of the sessions package, including modules base.py, client.py, and files.py could be restructured and streamlined. Rather than having the SessionEngine subclasses in separate modules, it would make more sense to simply move all of them into one engines.py module.

Along with that, The Session object, which is very alike to a dict, should be replaced by a dict subclass.