move want_replies into new plugin system

src/mesh/MeshPlugin.h

@@ -47,4 +47,10 @@ class MeshPlugin
     @return true if you've guaranteed you've handled this message and no other handlers should be considered for it
     */
     virtual bool handleReceived(const MeshPacket &mp) { return false; }
+
+    /** Messages can be received that have the want_response bit set.  If set, this callback will be invoked
+     * so that subclasses can (optionally) send a response back to the original sender.  Implementing this method
+     * is optional
+     */
+    virtual void sendResponse(NodeNum to) {}
 };