version the P2P protocol

version the P2P protocol

Unfortunately ReceiveMessage didn't handle unknown messages the way it
was documented to; client sending VERSION would cause the server to
return an ERROR and hang up. Fixed that, but old releases of git-annex
use the P2P protocol for tor and will still have that behavior.

So, version is not negotiated for Remote.P2P connections, only for
Remote.Git connections, which will support VERSION from their first
release. There will need to be a later flag day to change Remote.P2P;
left a commented out line that is the only thing that will need to be
changed then.

Version 1 of the P2P protocol is not implemented yet, but updated
the docs for the DATA change that will be allowed by that version.

This commit was sponsored by Jeff Goeke-Smith on Patreon.