Confierge - cache for post_config as well

If config saves fail due to connection issues, confierge should cache the new data similar to how it does for gets. This'll involve more complex logic to resolve such errors, as it's not as immediately clear how to automatically retry failed saves, especially with the potential for out-of-sync data. Like, should it retry periodically, or the next time you run the app? Or maybe just be loud about the failure and expect a human to fix it.