Revert "storage: make non-movable"

This reverts commit 7ebad7d3c5.

src/storage.h

@@ -91,6 +91,21 @@ struct Storage {
     Storage() {}
     Storage(time_point t) : maintenance_time(t) {}
 
+#if defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ <= 9 || defined(_WIN32)
+    // GCC-bug: remove me when support of GCC < 4.9.2 is abandoned
+    Storage(Storage&& o) noexcept
+        : maintenance_time(std::move(o.maintenance_time))
+        , listeners(std::move(o.listeners))
+        , local_listeners(std::move(o.local_listeners))
+        , listener_token(std::move(o.listener_token))
+        , values(std::move(o.values))
+        , total_size(std::move(o.total_size)) {}
+#else
+    Storage(Storage&& o) noexcept = default;
+#endif
+
+    Storage& operator=(Storage&& o) = default;
+
     bool empty() const {
         return values.empty();
     }
@@ -158,8 +173,6 @@ struct Storage {
 private:
     Storage(const Storage&) = delete;
     Storage& operator=(const Storage&) = delete;
-    Storage(Storage&& o) noexcept = delete;
-    Storage& operator=(Storage&& o) = delete;
 
     std::vector<ValueStorage> values {};
     size_t total_size {};