Currently, index builder votes for ready to commit after second drain phase. Now that first drain phase and second phase got split into 2 different methods by
SERVER-39458. And, SERVER-46560 has unified the code path for single phase & 2 phase index builds, it's good to move _signalPrimaryForCommitReadiness(voting logic) method after _insertKeysFromSideTablesWithoutBlockingWrites(first drain phase).
It should also make the behavior consistent with the simultaneous index build design doc.