Only retrieve one batch of messages at a time when responding to a large resend request

[philipwhiuk]

Fixes #639

[chrjohn]

@philipwhiuk do you happen to have the time to create a unit test for this? So we can ensure that normal resend processing works as expected? Probably a test where we resend more than 1000 messages and check that every message is resent. Something along that lines?
Thank you :)
Edit: call me paranoid but I'm always cautious when introducing new loops with seqnum+1... ;)