Fix to filter iterator busy bug

[timfox]

The bug that was fixed for the queue filter iterator busy issue, I notice that now if the consumer is busy the iterator gets reset to the beginning of the queue.

This seems inefficient to me, as if there are a lot of messages in the queue the iterator might have to scan all the way right through them the next time delivery is attempted.

A better solution would just be to cache the last message obtained from the iterator and represent that, the iterator itself wouldn't have to be reset.

[ataylor]

thats a good idea, i will make the changes

[timfox]

I've already made the changes.

I'm doing a lot of refactoring in this area, so I'd steer clear.