Simplify immutable download API: use just filenode.read(consumer, offset, size)

Simplify immutable download API: use just filenode.read(consumer, offset, size)

* remove Downloader.download_to_data/download_to_filename/download_to_filehandle
* remove download.Data/FileName/FileHandle targets
* remove filenode.download/download_to_data/download_to_filename methods
* leave Downloader.download (the whole Downloader will go away eventually)
* add util.consumer.MemoryConsumer/download_to_data, for convenience
  (this is mostly used by unit tests, but it gets used by enough non-test
   code to warrant putting it in allmydata.util)
* update tests
* removes about 180 lines of code. Yay negative code days!

Overall plan is to rewrite immutable/download.py and leave filenode.read() as
the sole read-side API.