Add CompletionAwareAggregationStrategy to the aggregator EIP

When people use the aggregator EIP, then they use the aggregation strategy interface.

What is needing sometimes is a callback when the aggregated exchange is done, so you can do any custom cleanup/logic there, before the exchange is being routed out of the aggregator.

Without this you would need to do this logic afterwards in the route, by calling a bean/processor. This is not so nice. So having a onComplete callback method in your aggregation strategy is better.