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/branch.go b/db/model/branch.go
index b59f727..ae0441a 100644
--- a/db/model/branch.go
+++ b/db/model/branch.go
@@ -19,14 +19,14 @@
 // TODO: missing proper logging
 
 type Branch struct {
-	Node      *Node
+	Node      *node
 	Txid      string
 	Origin    Revision
 	Revisions map[string]Revision
 	Latest    Revision
 }
 
-func NewBranch(node *Node, txid string, origin Revision, autoPrune bool) *Branch {
+func NewBranch(node *node, txid string, origin Revision, autoPrune bool) *Branch {
 	cb := &Branch{}
 	cb.Node = node
 	cb.Txid = txid