mod_client_certs: do not crash on plain sockets In some situations (e.g., reverse-proxied websocket), non-TLS sockets can be marked as secure, causing mod_client_certs to call the undefined method getpeercertificate and crash.

This module blocks configured queries that change server state.

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

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