VOL-2726 Changed all usages of childTypesSingleton to use the provided getters and setters.

This makes map access thread-safe.

Change-Id: I5cc1c6fa5c074c3020a1e1e37e88d2456f81ad7f
diff --git a/db/model/child_type_test.go b/db/model/child_type_test.go
index 5a64264..b57d988 100644
--- a/db/model/child_type_test.go
+++ b/db/model/child_type_test.go
@@ -39,10 +39,13 @@
 
 // Verify that the cache contains an entry for types on which ChildrenFields was performed
 func TestChildType_02_Cache_Keys(t *testing.T) {
-	if _, exists := getChildTypes().Cache[reflect.TypeOf(&voltha.Device{}).String()]; !exists {
-		t.Errorf("getChildTypeCache().Cache should have an entry of type: %+v\n", reflect.TypeOf(&voltha.Device{}).String())
+	childTypes.mutex.RLock()
+	defer childTypes.mutex.RUnlock()
+
+	if _, exists := childTypes.cache[reflect.TypeOf(&voltha.Device{})]; !exists {
+		t.Errorf("childTypes.cache should have an entry of type: %+v\n", reflect.TypeOf(&voltha.Device{}).String())
 	}
-	for k := range getChildTypes().Cache {
-		t.Logf("getChildTypeCache().Cache Key:%+v\n", k)
+	for k := range childTypes.cache {
+		t.Logf("childTypes.cache Key:%+v\n", k)
 	}
 }