clean up protocol version for proxying

clean up protocol version for proxying

The proxy always checks the protocol version of a remote before talking
to it in a version-specific way, so the protocol version in the ProxyParams
is the client's protocol version. The remote will always be at the same or
an older protocol version than the client.

Note that in relayDATAFinish, when the client is at protocol version 0,
the remote must thus be as well, and that's why its version is not
checked in the case for that.

With that clarified, it's evident that, in P2P.Http.State, there's no
need to look at the proxied remote's protocol version at all.