Collections: Prepares don't count towards collection disk size (they should)

When a prepare is flushed, we skip updating the owning collection's disk-size stat. Given that a prepare could be of the maximum document size (20Mb) a user could quickly use disk space, but have no idea which collection is using that space.

Fixing this primarily requires changes in compaction, where prepares are purged from the system and we would need to decrement collection disk-sizes for each purged prepare.