ldp-service-fs should be a concrete implementation of ldp-service/storage.js

[jamsden]

ldp-service-fs is a full implementation of ldp-service on the file system. It should be refactored to be a concrete implementation of ldp-service/storage.js on the file system.

It is not clear there is much need for this implementation other than to have another concrete implementation of storage.js abstract ldp-service storage needs to ensure the abstract specification is sufficient and handles all the commonality/variability that might arise for different concrete storage service implementations.