mod_sasl2_fast: Fix malformed-request when using HT-*-NONE (thanks lnj!) This crept into the previous commit which tried to fail early when CB was requested but unavailable - that commit did not actually check that CB *was* requested.

This module blocks configured queries that change server state.

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

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