Per the design document,

"A hook will be added to replSetReconfig to inform index builds of replica set config changes that could affect their ability to ever achieve their commitQuorum setting. replSetReconfig can remove replica set members, remove replica tags sets or change member voting rights. Index builds on primaries will check whether quorum can still be achieved and abort the build if not. Quorum achievability will also be checked on stepup, to eliminate races. Additionally, replSetReconfig can be called against a secondary, and the primary may only discover the reconfig via a replSetHeartbeat. So a similar hook will be added to the code replSetHeartbeat uses when the replset config version number changes, reflecting a config change, to inform the primary in that situation."

Add a new function, onReplicaReconfig, that the hooks call into. We'll leave the function unimplemented for now. Someday it will prompt index builds synchronously or asynchronously to check for commitQuorum satisfiability.