Complete the getter-based transformation, by hiding ".uri" and updating
callers to use get_uri(). Also don't set a dummy self._uri, leave it
undefined until someone calls set_uri().
* 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.
