Multi-Content Revisions/Dumps

The XML dump format will have to be changed to accommodate multiple slots per revision. Some considerations:


 * Some information that used to be associated with the revision will now be (only or also) be associated with each slot.
 * If possible, we want to avoid breaking clients.
 * For import, MediaWiki should support the old as well as the new format.

Straw man format:


 * should we continue to use nested tags for meta-data, or start using attributes?
 * can we use back-references for content that was already present in earlier revisions?
 * Dump generation relies on low-level access to metadata and content. RevisionSlotStore and BlobStore will probably be used here together, and will probably need a batch interface.