[thin_repair] speed up ref count validation

space_map.h

@@ -29,6 +29,16 @@ namespace persistent_data {
 		virtual bool count_possibly_greater_than_one(block_address b) const = 0;
 
 		virtual void extend(block_address extra_blocks) = 0;
+
+		struct iterator {
+			virtual ~iterator() {}
+
+			virtual void operator() (block_address b, ref_t c) = 0;
+		};
+
+		virtual void iterate(iterator &it) const {
+			throw std::runtime_error("not implemented");
+		}
 	};
 
 	class persistent_space_map : public space_map {