This is mostly a consistency addition, considering that most (if not
all) loaders will only add a single snapshot.
The common pattern of loading objects in topological order (content >
directory > revision > release > snapshot), then flushing the storage,
is now fully consistent; Without this addition, the snapshot addition
would reach the backend storage before all other objects are added,
leading to potential inconsistencies if the flush of other object types
fails.
Depends on D5013.