Photo id references for galleries not rewritten

[zpao]

When importing posts with galleries, the references to attachments aren't rewritten, sometimes resulting in the galleries containing the incorrect images. This likely isn't as much a problem when importing into a Wordpress instance - which is probably the more common case. The work I was doing required importing from multiple WXR files, with conflicting ids.

Featured images are already re-mapped so the data tracking is already done. The difference is that fixing this is not just fixing meta data, but modifying the content of posts.