Fix regressions in SimpleThreadScope and SimpleTransactionScope

PR gh-25038 introduced regressions in SimpleThreadScope and
SimpleTransactionScope in Spring Framework 5.2.7. Specifically, if a
thread-scoped or transaction-scoped bean has a dependency on another
thread-scoped or transaction-scoped bean, respectively, a
ConcurrentModificationException will be thrown on Java 11 or higher.

The reason is that Java 11 introduced a check for concurrent
modification in java.util.HashMap's computeIfAbsent() implementation,
and such a modification can occur when a thread-scoped bean is being
created in order to satisfy a dependency of another thread-scoped bean
that is currently being created.

This commit fixes these regressions by switching from HashMap to
ConcurrentHashMap for the instance maps in SimpleThreadScope and
SimpleTransactionScope.

Closes gh-25618

spring-context/src/main/java/org/springframework/context/support/SimpleThreadScope.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,8 +16,8 @@
 
 package org.springframework.context.support;
 
-import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -59,7 +59,7 @@ public class SimpleThreadScope implements Scope {
 			new NamedThreadLocal<Map<String, Object>>("SimpleThreadScope") {
 				@Override
 				protected Map<String, Object> initialValue() {
-					return new HashMap<>();
+					return new ConcurrentHashMap<>();
 				}
 			};