RSVP: passing up a message batch with some RSVP messages causes incorrect ordering

If we have a batch of messages 5-10 and message 7 is tagged as RSVP, then the logic in RSVP.up(MessageBatch) will do the following:

• Iterate through the batch
• If a message is marked as RSVP: remove it and pass it up (msg 7)
• The pass the batch up
• This means we'll pass up message 7 first, then messages 5,6,8,9,10
  --> Incorrect order

SOLUTION:

• We cannot pass up RSVP messages directly
• Instead, remember the RSVP IDs of the messages in the batch which were marked as RSVP
• When done, send an RSVP response for each of these IDs