[close #489 ] fix backoffer data race (#490) (#494)

* cherry pick #490 to release-3.1

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

* fix license error

Signed-off-by: iosmanthus <myosmanthustree@gmail.com>

Co-authored-by: iosmanthus <dengliming@pingcap.com>
Co-authored-by: iosmanthus <myosmanthustree@gmail.com>

src/test/java/org/tikv/util/ConcreteBackOfferTest.java

@@ -1,16 +1,18 @@
 /*
- * Copyright 2017 PingCAP, Inc.
+ * Copyright 2021 TiKV Project Authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
- *      http://www.apache.org/licenses/LICENSE-2.0
+ * http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
+ *
  */
 
 package org.tikv.util;