mod_turncredentials: Add 'transport' attribute XEP-0215 recommends using 'transport' attribute and since coturn can work in both TCP and UPD modes this patch adds one <service> element for each combination of STUN/TURN and TCP/UDP.

This module blocks configured queries that change server state.

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

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