Factor out commonality between WorkerProto::Basic{Client,Server}Connection

This also renames clientVersion and daemonVersion to the more correct
protoVersion (since it's the version agreed to by both sides).

src/libstore/worker-protocol.hh

@@ -82,6 +82,7 @@ struct WorkerProto
      *
      * @todo remove once Hydra uses Store abstraction consistently.
      */
+    struct BasicConnection;
     struct BasicClientConnection;
     struct BasicServerConnection;