ARTEMIS-4972 Use Lock in start() for Binding Variables

artemis-server/src/main/java/org/apache/activemq/artemis/core/server/group/impl/LocalGroupingHandler.java

@@ -322,10 +322,16 @@ public final class LocalGroupingHandler extends GroupHandlingAbstract {
       if (started)
          return;
 
+      lock.lock();
+
+      try {
          if (expectedBindings == null) {
             // just in case the component is restarted
             expectedBindings = new LinkedList<>();
          }
+      } finally {
+         lock.unlock();
+      }
 
       if (reaperPeriod > 0 && groupTimeout > 0) {
          if (reaperFuture != null) {