Two phase index builds should be able to keep running on the IndexBuildsCoordinator's thread pool across replica set state transitions such as stepup and stepdown. In the case of a primary to secondary transition, an index build should not conflict with oplog application post step-down. To support this scenario, the index build should explicitly opt out of the PBWM at the beginning of the build (since the behavior for PBWM is opt-in by default).