The ocis storage driver - a deconstructed filesystem

<!--HTML-->Looking up files by a stable id is an inefficient operation in most filesystems. While an efficient lookup by file id can be cached inside an OCIS storage provider this cache needs to be kept up to date. By deconstructing a filesystem and storing every node by its uuid we can evade the cache invalidation problem at the cost of more stat requests. The ocis storage driver allows an efficient lookup of file metadata by path and by file id while relying solely on the filesystem as a persistence layer. Furthermore, the layout on disk can be used to implement trash, versions and in the future deduplication. It serves as a blueprint for seperating metadata and content in an S3 or librados storage driver.