[FLINK-14602][runtime] Change Type of Field tasks from ConcurrentHash…

…Map to HashMap

This closes apache#10101.

flink-runtime/src/main/java/org/apache/flink/runtime/executiongraph/ExecutionGraph.java

@@ -193,7 +193,7 @@ public class ExecutionGraph implements AccessExecutionGraph {
 	private boolean isStoppable = true;
 
 	/** All job vertices that are part of this graph. */
-	private final ConcurrentHashMap<JobVertexID, ExecutionJobVertex> tasks;
+	private final Map<JobVertexID, ExecutionJobVertex> tasks;
 
 	/** All vertices, in the order in which they were created. **/
 	private final List<ExecutionJobVertex> verticesInCreationOrder;
@@ -462,7 +462,7 @@ public ExecutionGraph(
 			allocationTimeout);
 		this.userClassLoader = Preconditions.checkNotNull(userClassLoader, "userClassLoader");
 
-		this.tasks = new ConcurrentHashMap<>(16);
+		this.tasks = new HashMap<>(16);
 		this.intermediateResults = new HashMap<>(16);
 		this.verticesInCreationOrder = new ArrayList<>(16);
 		this.currentExecutions = new ConcurrentHashMap<>(16);