Offline GetWallpapers request

[vkryl]

Mobile clients cache wallpapers list locally. Currently TDLib has no cache in GetWallpapers request. This somewhat limits UI implementation of wallpaper selection (e.g. on iOS client it displayed top wallpapers in settings without connection – in case of TDLib it would require making request & response wait on each application start).

TDLib could, for example, always return local list of wallpapers immediately (if there's one), make server request asynchronously once in a day or a couple when GetWallpapers get called, then send updateWallpapers in rare cases when this list has changed.