Log Message:
Update to 1.9.0.

- The most significant new feature in this release is MDMF: "Medium-size
  Distributed Mutable Files". Unlike standard SDMF files, these provide
  efficient partial-access (reading and modifying small portions of the file
  instead of the whole thing). MDMF is opt-in (it is not yet the default
  format for mutable files), both to ensure compatibility with previous
  versions, and because the algorithm does not yet meet memory-usage goals.
  Enable it with ``--format=MDMF`` in the CLI (``tahoe put`` and ``tahoe
  mkdir``), or the "format" radioboxes in the web interface. See
  `<docs/specifications/mutable.rst>`_ for more details (`#393`_, `#1507`_)
- A "blacklist" feature allows blocking access to specific files through
  a particular gateway. See the "Access Blacklist" section of
  `<docs/configuration.rst>`_ for more details. (`#1425`_)
- A "drop-upload" feature has been added, which allows you to upload
  files to a Tahoe-LAFS directory just by writing them to a local
  directory. This feature is experimental and should not be relied on
  to store the only copy of valuable data. It is currently available
  only on Linux. See `<docs/frontends/drop-upload.rst>`_ for documentation.
  (`#1429`_)
- The timeline of immutable downloads can be viewed using a zoomable and
  pannable JavaScript-based visualization. This is accessed using the
  'timeline' link on the File Download Status page for the download, which
  can be reached from the Recent Uploads and Downloads page.