Support non-UTF8 blobs and consistent support for watches across backends

• More consistent support for notifications. irmin watch works
  now for all backends.
• Support for different blob formats on the command-line
• Support for JSON blobs
• More flexible irmin fetch command: we can now choose the backend to
  import the data in
• Fix import of Git objects when the blobs were not imported first
• Support non-UTF8 strings as path name and blob contents (for all
  backends, including the JSON one)
• Speed-up the slow tests execution time
• Improve the output graph when objects of different kinds might have
  the same SHA1