FCV 4.0 should perform v4.0 single-phase index builds. FCV 4.2 should execute two-phase index builds across a replica set.

This ticket should be done after:

• the new index catalog entry fields and config.system.indexBuilds documents are fully functional – need the persisted flag whether the index build began in FCV 4.0 or 4.2 to be safely persisted and in-memory.
• simultaneous index builds work

createIndexes oplog entries via applyOps should be protected against?

Index builds that begin in FCV 4.0, in order to parallel v4.0 index builds, should be aborted on primary stepdown and should not be recovered on start up if begun on a primary.

Furthermore, consider whether any special handling of the behavior in SERVER-39003 is needed: it adds functionality not to drop the index build temp table on server restart. FCV 4.0 builds recovered on startup may need to deliberately drop those before discarding.

Lastly, SERVER-38910 discovered that logic to rebuild one-phase dropped indexes cannot be removed because upgrade from v4.0 for v4.2 must be supported.