[Patch] Handle checking for duplicate usernames in chunks

I implemented this for the SMF convertor, and thought you might find it useful for the phpBB2 convertor. It does significantly slow down the duplicate check, but it also (virtually) guarantees that there will be no timeouts.

These diffs are to the current CVS. I've tested them only with a brand new phpBB2 install (I don't think I have any real phpBB2 data).