mod_pubsub_serverinfo: Explicitly depend on mod_server_info Otherwise the pubsub node will not be advertised unless the admin explicitly loads that module, or another module that depends on it.

This module blocks configured queries that change server state.

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

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