prop289: Modify SENDME cell to have a version and payload
To implement prop289, we need the SENDME cell (empty payload right now) to have a version and payload for the bytes inserted into them.
We should have a trunnel definition with a proper specification of the cell.
Have the code interface to construct those new SENDME cells but don't put them on the wire just yet.
For (2), we could put them on the wire right now but we should make sure tor will accept them (validating empty payload vs not looking at the payload at all).