Refactor Introduce messages to support looking at encoded headers

Our hs_ntor handshake uses the text of these headers as a sort of "extra data" input to its encryption. Thus we need to be able to get the text of a header before we finish making the whole Introduce1 message, and we need to be able to preserve their text of an Introduce2 message's header after decoding it, so that we can decrypt the encrypted portion of it.

Closes #866 (closed).