Filesystem storage driver

An implementation of the storagedriver.StorageDriver interface which uses the local filesystem.

• rootdirectory: (optional) The absolute path to a root directory tree in which to store all registry files. The registry stores all its data here so make sure there is adequate space available. Defaults to /var/lib/registry. If the directory does not exist, it will be created honoring umask bits. If umask bits are not set, the resulting permission will be 0777.
• maxthreads: (optional) The maximum number of simultaneous blocking filesystem operations permitted within the registry. Each operation spawns a new thread and may cause thread exhaustion issues if many are done in parallel. Defaults to 100, and cannot be lower than 25.