Make const overload of ClientData::Site::ForEach usable

src/ClientData.h

@@ -416,7 +416,12 @@ private:
    {
       return Locked< DataContainer >{ mData };
    }
-
+   
+   Locked<const DataContainer> GetData() const
+   {
+      return Locked< const DataContainer >{ mData };
+   }
+   
    static void EnsureIndex( Locked<DataContainer> &data, size_t index )
    {
       if (data.mObject.size() <= index)

src/ClientDataHelpers.h

@@ -57,19 +57,19 @@ template< typename Object > struct Lockable< Object, NoLocking >
 : Object {
    // implement trivial non-locking policy
    struct Lock{};
-   Lock lock() { return {}; }
+   Lock lock() const { return {}; }
 };
 template< typename Object > struct Lockable< Object, NonrecursiveLocking >
 : Object, std::mutex {
    // implement real locking
    using Lock = std::unique_lock< std::mutex >;
-   Lock lock() { return Lock{ *this }; }
+   Lock lock() const { return Lock{ *this }; }
 };
 template< typename Object > struct Lockable< Object, RecursiveLocking >
 : Object, std::recursive_mutex {
    // implement real locking
    using Lock = std::unique_lock< std::recursive_mutex >;
-   Lock lock() { return Lock{ *this }; }
+   Lock lock() const { return Lock{ *this }; }
 };
 
 // Pairing of a reference to a Lockable and a lock on it