In SERVER-96495, we added logic to prefer syncing from the primary if we are choosing between the primary and another node in a datacenter.

The check is done using member data of each node. However, this data may be stale, since we mark a node as primary if they self report primary. Then, we use _currentPrimaryIndex as our final index. We may be running this code as part of primary catchup, which then causes us to set our sync source index to ourself.

This later triggers an invariant here when we re-evaluate sync sources.