Check if storage engine supports Recover to Stable Timestamp rather than if eMRC=false in sync_tail.cpp

We invariant if eMRC is not false here because we assume that it's the only situation where the node has completed a rollback via refetch. But on 4.0 if the node is using an inMemory storage engine, it will also not support recover to a stable timestamp, meaning the node could have gone through a rollback via refetch.