INT-4428: MethodParam usage: fix race condition

JIRA: https://jira.spring.io/browse/INT-4428

Using `increaseNestingLevel()` and `decreaseNestingLevel()` is not
thread-safe and may cause a race conditions

* Use `MethodParameter.nested()` instead which creates and cache a new
`MethodParameter` for the nested generic type

**Cherry-pick to 5.0.x**

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/handler/support/CollectionArgumentResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 the original author or authors.
+ * Copyright 2017-2018 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.
@@ -65,7 +65,7 @@ public boolean supportsParameter(MethodParameter parameter) {
 
 	@Override
 	@SuppressWarnings("unchecked")
-	public Object resolveArgument(MethodParameter parameter, Message<?> message) throws Exception {
+	public Object resolveArgument(MethodParameter parameter, Message<?> message) {
 		Object value = message.getPayload();
 
 		if (this.canProcessMessageList) {
@@ -75,16 +75,14 @@ public Object resolveArgument(MethodParameter parameter, Message<?> message) thr
 
 			Collection<Message<?>> messages = (Collection<Message<?>>) value;
 
-			parameter.increaseNestingLevel();
-			if (Message.class.isAssignableFrom(parameter.getNestedParameterType())) {
+			if (Message.class.isAssignableFrom(parameter.nested().getNestedParameterType())) {
 				value = messages;
 			}
 			else {
 				value = messages.stream()
 						.map(Message::getPayload)
 						.collect(Collectors.toList());
 			}
-			parameter.decreaseNestingLevel();
 		}
 
 		if (Iterator.class.isAssignableFrom(parameter.getParameterType())) {

spring-integration-core/src/main/java/org/springframework/integration/util/MessagingMethodInvokerHelper.java

@@ -1241,9 +1241,7 @@ private void setExclusiveTargetParameterType(TypeDescriptor targetParameterType,
 			}
 			this.targetParameterTypeDescriptor = targetParameterType;
 			if (Message.class.isAssignableFrom(targetParameterType.getObjectType())) {
-				methodParameter.increaseNestingLevel();
-				this.targetParameterType = methodParameter.getNestedParameterType();
-				methodParameter.decreaseNestingLevel();
+				this.targetParameterType = methodParameter.nested().getNestedParameterType();
 			}
 			else {
 				this.targetParameterType = targetParameterType.getObjectType();