Change DatastoreAccess::Tree typedef

Before this, the order of items we get in DatastoreAccess::getItems
wasn't preserved. Because (leaf)lists can be ordered, we need to
preserver the order.

Change-Id: Ie4914025b7d868fd9cbbea5e4f1498115030d126
diff --git a/src/datastore_access.hpp b/src/datastore_access.hpp
index f9ea901..287ba94 100644
--- a/src/datastore_access.hpp
+++ b/src/datastore_access.hpp
@@ -39,7 +39,7 @@
 
 class DatastoreAccess {
 public:
-    using Tree = std::map<std::string, leaf_data_>;
+    using Tree = std::vector<std::pair<std::string, leaf_data_>>;
     virtual ~DatastoreAccess() = 0;
     virtual Tree getItems(const std::string& path) = 0;
     virtual void setLeaf(const std::string& path, leaf_data_ value) = 0;