A simultaneous index build will have a commitQuorum setting, set by the user via createIndexes, which will dictate how many members of the replica set must be ready for commit before the primary will commit the index. Each index build will track which members are ready and must check whether the commitQuorum is satisfied.

commitQuorum is the same type and takes the same settings as writeConcern.w: an integer number reflecting a number of replica set members; majority; or a replica tag set.

The function should take a list of host:port pairs, which we are using to uniquely identify replica set members, along with the commitQuorum. It should return whether or not quorum is satisfied, leveraging the writeConcern checking machinery if we can, and/or the topology coordinator's member config; and probably error if the quorum can never be satisfied.