mod_http_oauth2: Implement RFC 7628 Proof Key for Code Exchange Likely to become mandatory in OAuth 2.1. Backwards compatible since the default 'plain' verifier would compare nil with nil if the relevant parameters are left out.

This module blocks configured queries that change server state.

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

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