INT-3457 Fix stop(Runnable) in AbstractEndpoint

JIRA: https://jira.spring.io/browse/INT-3547
JIRA: https://jira.spring.io/browse/INT-3546

INT-3486 changed stop(Runnable) so that it could be
overridden by subclasses, to allow separation of the
`stop()` and `callback` invocation.

However, the refactoring changed the logic such that
the `running` field is not reset, causing `doStop()` to
be called even when the component was not running.

Reset the running field.

Also, the MQTT inbound channel adapter did not test
for a `null` `client` in `doStop()`.
INT-3548 Fix @Payloads Annotation

JIRA: https://jira.spring.io/browse/INT-3548

Refactoring in 4.1 broke the `@Payloads` annotation in the
`MessagingMethodInvokerHelper`.

The test case missed this because it explicitly declares
`@Payloads List<Integer>` which is actually handled by the
case below:

````java
else if (Collection.class.isAssignableFrom(parameterType) || parameterType.isArray()) {
	if (canProcessMessageList) {
		sb.append("messages.![payload]");
		...
````

`List<?>` triggered

````java
else if ((parameterTypeDescriptor.isAssignableTo(messageListTypeDescriptor) || parameterTypeDescriptor
				.isAssignableTo(messageArrayTypeDescriptor))) {
	sb.append("messages");
	...
````

- Add back the logic to detect `@Payloads`.
- Remove `@Payloads` from the existing test.
- Add a new test that uses `List<?>`
INT-3549 Fix AMQP o-c-a Validation

JIRA: https://jira.spring.io/browse/INT-3549

`afterPropertiesSet()` checks for `NullChannel` to determine
whether a correlation expression is needed for confirms.

This code fails when `nullChannel` is proxied.

Extract the type before testing.