title: Spring Integration - ReleaseStrategy that looks at messages that weren't yet added
body: <p>I have messages that are fetched to the system in groups (let's say 50), need to be grouped by AGGREGATION_ID into lists of messages and send further into the flow.</p>
  
  <p>I can use correlationStrategy to aggregate with that id, but I need to know when to release the aggregated message. In ReleaseStrategy I can only look at the messages already added to the aggregate, but I need to know when there's no more messages in the fetched group of 50 with the same AGGREGATION_ID to know when to send the group. How can I do that?</p>