We set index builds to ignore prepare conflicts using the kIgnoreConflictsAllowWrites setting, which appears to only be used to set a flag on a WT request, but then we also enforce that internal threads can be killed for step-down in order to avoid hangs with said threads hitting prepare conflicts, without any apparent exception for threads that have kIgnoreConflictsAllowWrites set.

I believe there should be an exception to the client->canKillSystemOperationInStepdown() invariant where internal threads with kIgnoreConflictsAllowWrites set should not be required to be killable.