We should have all the pieces in place for this now. IndexBuildsCoordinator::setCommitQuorum should return an error if the new commitQuorum cannot be satisfied. If it is satisfiable, then it should be persisted. I think unit testing should be sufficient: JS testing would work, too, but doesn't provide anything over unit testing at the moment, until simul index builds start using commitQuorum, I think.

We'll eventually need to figure out how to notify simul index builds of the new commitQuorum, but we don't have enough pieces in place yet to see how it would come together. If it becomes clear, we can make another ticket for that work.