public void clear() {
    lock.writeLock().lock();
    try {
      delegate.clear();
    } finally {
      lock.writeLock().unlock();
    }
  }
Atomically removes all of the elements from this queue. The queue will be empty after this call
returns.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/normalizer/RegionNormalizerWorkQueue.java