misc/munin/prosody_: Add a total_s2s counter to work with mod_statistics_statsman mod_s2s doesn't currently count s2sin and s2sout separately, so this is a compromise for now.

This module blocks configured queries that change server state.

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

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