finish P2P protocol proxying

finish P2P protocol proxying

CONNECT is not supported by git-annex-shell p2pstdio, but for proxying
to tor-annex remotes, it will be supported, and will make a git pull/push
to a proxied remote work the same with that as it does over ssh,
eg it accesses the proxy's git repo not the proxied remote's git repo.

The p2p protocol docs say that NOTIFYCHANGES is not always supported,
and it looked annoying to implement it for this, and it also seems
pretty useless, so make it be a protocol error. git-annex remotedaemon
will already be getting change notifications from the proxy's git repo,
so there's no need to get additional redundant change notifications for
proxied remotes that would be for changes to the same git repo.