fix ConcurrentModificationException by replacing HashMap with ConcurrentHashMap (thulab#476)

Author: YangYumings

core/src/main/java/cn/edu/tsinghua/iot/benchmark/mode/BaseMode.java

@@ -35,9 +35,9 @@
 
 import java.util.ArrayList;
 import java.util.Collections;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.CyclicBarrier;
 import java.util.concurrent.ExecutionException;
@@ -285,7 +285,7 @@ private static void measure(
     }
   }
 
-  private static Map<Operation, String> sqlMap = new HashMap<>();
+  private static Map<Operation, String> sqlMap = new ConcurrentHashMap<>();
 
   /**
    * Each type of query is recorded once.