negotiate P2P protocol version for tor remotes

This negotiation is not supported by versions of git-annex older
than 6.20180312. Well, maybe really 6.20180227 or so, but using that in
the changelog simplifies things since it was the version for the other
changes as well.

See commit c81768d425 for the back story.

As well as allowing for future protocol improvements, this will result
in negoatiating protocol version 1, which is an improvement over default
version 0.

In fact, it looks like no supported version of git-annex will use
protocol version 0, since version 1 was introduced in 6.20180227.
Still, removing the code for version 0 seems unncessary.
See commit 31e1adc005.

Sponsored-by: Brett Eisenberg on Patreon.

Remote/P2P.hs

@@ -148,14 +148,7 @@ openConnection u addr = do
 			authtoken <- fromMaybe nullAuthToken
 				<$> loadP2PRemoteAuthToken addr
 			let proto = P2P.auth myuuid authtoken $
-				-- Before 6.20180312, the protocol server
-				-- had a bug that made negotiating the
-				-- protocol version terminate the
-				-- connection. So, this must stay disabled
-				-- until the old version is not in use
-				-- anywhere.
-				--P2P.negotiateProtocolVersion P2P.maxProtocolVersion
-				return ()
+				P2P.negotiateProtocolVersion P2P.maxProtocolVersion
 			runst <- liftIO $ mkRunState Client
 			res <- liftIO $ runNetProto runst conn proto
 			case res of