Improve publishing performance by using virtual graphs / rewriting everywhere

[ricroberts]

(Long term) idea to address publishing perf:

• store additions and deletions in virtual graphs, and apply rewriting everywhere to UNION/MINUS them up (even in live).
• We can use batching to publish changes, as we can only change the live endpoint to include the new graph when we're ready.
• We'll prob need to periodicially squash the changes in the background so the rewritten queries don't get too crazy. This can prob happen batched too (as we don't need to 'commit' it to live until we're ready).
• We'll need to make sure rewriting is perfect (or as good as).
• We'll need to check how performant doing rewriting like this is!