Name the protocol version types

This makes the code clearer, and will help us replace them with proper
structs and get rid of the macros later.

src/libstore/serve-protocol.hh

@@ -30,6 +30,13 @@ struct ServeProto
      */
     enum struct Command : uint64_t;
 
+    /**
+     * Version type for the protocol.
+     *
+     * @todo Convert to struct with separate major vs minor fields.
+     */
+    using Version = unsigned int;
+
     /**
      * A unidirectional read connection, to be used by the read half of the
      * canonical serializers below.