def start():
    """Begin the encode/upload process. This involves reading encrypted
        data from the IEncryptedUploadable, encoding it, uploading the shares
        to the shareholders, then sending the hash trees.

        set_encrypted_uploadable() and set_shareholders() must be called
        before this can be invoked.

        This returns a Deferred that fires with a verify cap when the upload
        process is complete. The verifycap, plus the encryption key, is
        sufficient to construct the read cap.
        """
