immutable: refactor download to do only download-and-decode, not decryption

FileDownloader takes a verify cap and produces ciphertext, instead of taking a read cap and producing plaintext.
FileDownloader does all integrity checking including the mandatory ciphertext hash tree and the optional ciphertext flat hash, rather than expecting its target to do some of that checking.
Rename immutable.download.Output to immutable.download.DecryptingOutput. An instance of DecryptingOutput can be passed to FileDownloader to use as the latter's target.  Text pushed to the DecryptingOutput is decrypted and then pushed to *its* target.
DecryptingOutput satisfies the IConsumer interface, and if its target also satisfies IConsumer, then it forwards and pause/unpause signals to its producer (which is the FileDownloader).
This patch also changes some logging code to use the new logging mixin class.
Check integrity of a segment and decrypt the segment one block-sized buffer at a time instead of copying the buffers together into one segment-sized buffer (reduces peak memory usage, I think, and is probably a tad faster/less CPU, depending on your encoding parameters).
Refactor FileDownloader so that processing of segments and of tail-segment share as much code is possible.
FileDownloader and FileNode take caps as instances of URI (Python objects), not as strings.

src/allmydata/client.py

@@ -361,7 +361,7 @@ class Client(node.Node, pollmixin.PollMixin):
                 else:
                     key = base32.b2a(u.storage_index)
                     cachefile = self.download_cache.get_file(key)
-                    node = FileNode(u.to_string(), self, cachefile) # CHK
+                    node = FileNode(u, self, cachefile) # CHK
             else:
                 assert IMutableFileURI.providedBy(u), u
                 node = MutableFileNode(self).init_from_uri(u)