Update `XvcStorage` trait functions to receive absolute cache directory instead of `XvcRoot`

[Emre Sahin]

• Instead of xvc_root, it will be used to generate upload/download paths.
• We can pass a temp directory (for receive) or pass the cache directly (for send).
• xvc file send and xvc file bring will be decoupled from xvc storage.
  This will enable to use the same machinery for e.g. experiments.