There are currently two ways that the stable component of a layered table can be created. The first is on the primary, where it's a normal session->create operation.

The second way is when the secondary steps up to a primary after seeing a table creation from the oplog, and we need to create the "missing" stable component.

These two cases use pretty much the same config string each time, and I think it's reasonable to refactor these two locations into one.