INT-3528 Use Collections min and max methods where appropriate

JIRA: https://jira.spring.io/browse/INT-3528

Use `min` and `max` methods instead of sorting an entire collection to get the minimum or maximum element.

Change other inline `Comparator<?>`s to the `final` fields

Conflicts:
	spring-integration-core/src/main/java/org/springframework/integration/aggregator/AbstractCorrelatingMessageHandler.java