There are certain places where we set the WriteUnitOfWork's oplog grouping type to kDontGroup because that was the assumed default however after SERVER-114334, we will flip the oplog grouping type to kGroupForPossiblyRetryableOperations in certain cases. This makes it complicated if we actually want to use kDontGroup as the oplog grouping type.

If we introduce a kDefault grouping type, we make it explicit that we just want the default behavior and it will be clearer if other callers actually want certain grouping types.