Store leaves in _attachments for smaller doc, better view performance

[Jason Gerard DeRose]

The leaves for the hash-list implemented in lp:704272 are currently base32-encoded and stored in the 'leaves' list in the document.

However, we will never be indexing based on the leave hashes, so this is a lot of unnecessary data getting sent to the view server, so the leaf hashes would be much better store in _attachments. Also, the leave values are never going to be used by, say, the HTML5 UI, so again, unnecessary data and json encoding/decoding.

So the leaves will be stored as an attachment. The binary digests will be concatenated together, which will keep the CouchDB database size a bit small also.