Fix GetAllChildReferences used by migration filter

Migration programs in onflow/flow-go added a flag to filter
old unreferenced slabs and onflow/atree added some functions
to support that. However, some of the old unreferenced slabs
are not filtered during migration.

This commit fixes the migration filter by handling nested
storage ID inside element such as Cadence SomeValue.

storage.go

@@ -1071,40 +1071,60 @@ func (s *PersistentSlabStorage) FixLoadedBrokenReferences(needToFix func(old Val
 			return false
 		}
 
-		var isMetaDataSlab bool
-
 		switch slab.(type) {
-		case *ArrayMetaDataSlab, *MapMetaDataSlab:
-			isMetaDataSlab = true
-		}
+		case *ArrayMetaDataSlab, *MapMetaDataSlab: // metadata slabs
+			var foundBrokenRef bool
 
-		var foundBrokenRef bool
-		for _, childStorable := range slab.ChildStorables() {
+			for _, childStorable := range slab.ChildStorables() {
 
-			storageIDStorable, ok := childStorable.(StorageIDStorable)
-			if !ok {
-				continue
-			}
+				if slabIDStorable, ok := childStorable.(StorageIDStorable); ok {
 
-			childID := StorageID(storageIDStorable)
+					childID := StorageID(slabIDStorable)
 
-			// Track parent-child relationship of root slabs and non-root slabs.
-			if isMetaDataSlab {
-				parentOf[childID] = id
-			}
+					// Track parent-child relationship of root slabs and non-root slabs.
+					parentOf[childID] = id
 
-			if s.existIfLoaded(childID) {
-				continue
+					if !s.existIfLoaded(childID) {
+						foundBrokenRef = true
+					}
+
+					// Continue with remaining child storables to track parent-child relationship.
+				}
 			}
 
-			foundBrokenRef = true
+			return foundBrokenRef
+
+		default: // data slabs
+			childStorables := slab.ChildStorables()
+
+			for len(childStorables) > 0 {
+
+				var nextChildStorables []Storable
+
+				for _, childStorable := range childStorables {
+
+					if slabIDStorable, ok := childStorable.(StorageIDStorable); ok {
 
-			if !isMetaDataSlab {
-				return true
+						if !s.existIfLoaded(StorageID(slabIDStorable)) {
+							return true
+						}
+
+						continue
+					}
+
+					// Append child storables of this childStorable to
+					// handle nested StorageIDStorable, such as Cadence SomeValue.
+					nextChildStorables = append(
+						nextChildStorables,
+						childStorable.ChildStorables()...,
+					)
+				}
+
+				childStorables = nextChildStorables
 			}
-		}
 
-		return foundBrokenRef
+			return false
+		}
 	}
 
 	var brokenStorageIDs []StorageID
@@ -1273,6 +1293,11 @@ func (s *PersistentSlabStorage) getAllChildReferences(slab Slab) (
 
 			storageIDStorable, ok := childStorable.(StorageIDStorable)
 			if !ok {
+				nextChildStorables = append(
+					nextChildStorables,
+					childStorable.ChildStorables()...,
+				)
+
 				continue
 			}