NIFI-7576 Make http server init synchronized

so that multiple threads cannot enter `createHttpServerFromService`
simultaneously

nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/ListenHTTP.java

@@ -239,7 +239,7 @@ private void shutdownHttpServer(Server toShutdown) {
         }
     }
 
-    private void createHttpServerFromService(final ProcessContext context) throws Exception {
+    synchronized private void createHttpServerFromService(final ProcessContext context) throws Exception {
         if(initialized.get()) {
             return;
         }