ARTEMIS-908: Hold connection lock when issuing credits

artemis-protocols/artemis-amqp-protocol/src/main/java/org/apache/activemq/artemis/protocol/amqp/broker/AMQPSessionCallback.java

@@ -433,11 +433,13 @@ public class AMQPSessionCallback implements SessionCallback {
          store.checkMemory(new Runnable() {
             @Override
             public void run() {
+               synchronized (connection.getLock()) {
                   if (receiver.getRemoteCredit() < threshold) {
                      receiver.flow(credits);
                      connection.flush();
                   }
                }
+            }
          });
       } catch (Exception e) {
          throw new RuntimeException(e);