Prepare tor-cell API for proposal 340

After proposal 340 is done, RelayCell will no longer be a straightforward container for a single RelayMsg, and RelayCell will no longer exist in the same relationship to stream_id.

Since we're making breaking changes in the tor-cell API for #525 (closed), we may as well see if we can figure out a set of changes we can make now to prepare for the changes in proposal 340.

(This ticket is not for a complete implementation of prop340: just for the necessary preliminary changes.)