Improves performance of user directory migration

10x faster and uses 1/4 of the memory (~40MB to ~10MB)

Not pulling back full entities saved all the memory and made it 3x faster. Upping the chunk size increased the speed another 3x. I don't see why we couldn't up the chunk size again.

This was on a clean, default 1.8 site with 2000 users and no files.