MXS-2218 Centralize worker shutdown handling

server/core/mainworker.cc

@@ -45,6 +45,12 @@ MainWorker::~MainWorker()
     this_unit.pCurrent_main = nullptr;
 }
 
+//static
+bool MainWorker::created()
+{
+    return this_unit.pCurrent_main ? true : false;
+}
+
 //static
 MainWorker& MainWorker::get()
 {