Move ImmutableShare to backend specific module, add "backend" interface to Storage Server, use twisted's FilePath

[Zancas]

As a first step in implementing a pluggable backend, move ImmutableShare? to a module that is specific to the Disk backend. The Disk backend is the backend that corresponds to the current default storage medium. StorageServer? no longer communicates directly with the filesystem for purposes of setting up storage, instead it communicates with a backend "core" object that is specific to the type of storage. A Core class lives in each backend specific module along with the corresponding ImmutableShare?.