immutable repairer

immutable repairer
This implements an immutable repairer by marrying a CiphertextDownloader to a CHKUploader.  It extends the IDownloadTarget interface so that the downloader can provide some metadata that the uploader requires.
The processing is incremental -- it uploads the first segments before it finishes downloading the whole file.  This is necessary so that you can repair large files without running out of RAM or using a temporary file on the repairer.
It requires only a verifycap, not a readcap.  That is: it doesn't need or use the decryption key, only the integrity check codes.
There are several tests marked TODO and several instances of XXX in the source code.  I intend to open tickets to document further improvements to functionality and testing, but the current version is probably good enough for Tahoe-1.3.0.