File backend

A file backend based on the huebds library for manipulating data-store files should be added. This needs to have a compatibility layer so it can read files written with the commercial library (translating the serialized objects into JSON that is similar to the objects found in an object store version of VDS). The handling of chunk-metadata and metadata-pages is done quite differently for the file format, so there is some refactoring work to be done to make this backend possible.