mod_http_oauth2: Gracefully handle cache write failure Would previously have thrown an error and probably returned a traceback. This would only happen if a *lot* of authorization codes were requested in a short interval.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```