VOL-1255: Fixed an issue with the proxy implementation

- Had to change the model slightly to mimic a polymorphic behaviour

Change-Id: I12017cdfedf5c0ed05243245aa2a811556222e0f
diff --git a/db/model/non_persisted_revision.go b/db/model/non_persisted_revision.go
index 8b81a48..99166e5 100644
--- a/db/model/non_persisted_revision.go
+++ b/db/model/non_persisted_revision.go
@@ -89,7 +89,7 @@
 	return npr.Config.Data
 }
 
-func (npr *NonPersistedRevision) GetNode() *Node {
+func (npr *NonPersistedRevision) GetNode() *node {
 	return npr.Branch.Node
 }