R1:3e7b63aa736a

Add a <reviewer, revision> key to the reviewers table

Summary:
Ref T10967. I'm not 100% sure we need this, but the old edge table had it and I recall an issue long ago where not having this key left us with a bad query plan.

Our data doesn't really provide a way to test this key (we have many revisions and few reviewers, so the query planner always uses revision keys), and building a convincing test case would take a while (lipsum needs some improvements to add reviewers). But in the worst case this key…