add assertIsSetup() and use it from OSThread constructor

fixes nasty bug @mc-hamster discovered with plugin order of operations

src/plugins/ReplyPlugin.cpp

@@ -5,9 +5,6 @@
 
 #include <assert.h>
 
-// Create an a static instance of our plugin - this registers with the plugin system
-ReplyPlugin replyPlugin;
-
 MeshPacket *ReplyPlugin::allocReply()
 {
     assert(currentRequest); // should always be !NULL