mod_roster_command: Fix subscription when the "user JID" is a bare domain. Do not attempt to update the roster when the user is bare domain (e.g. a component), since they don't have rosters and the attempt results in an error: $ prosodyctl mod_roster_command subscribe proxy.example.com contact@example.com xxxxxxxxxxFailed to execute command: Error: /usr/lib/prosody/core/rostermanager.lua:104: attempt to concatenate local 'username' (a nil value) stack traceback: /usr/lib/prosody/core/rostermanager.lua:104: in function 'load_roster' /usr/lib/prosody/core/rostermanager.lua:305: in function 'set_contact_pending_out' mod_roster_command.lua:44: in function 'subscribe'

A module providing a streaming statistics interface like
[mod_statistics] but based on the new [statistics API][doc:statistics]
introduced in Prosody 0.10.

# Usage

To use, enable the built-in statistics like so:

```lua
statistics = "internal"
```

Then, in `modules_enabled`, replace `"statistics"` with
`"statistics_statsman"` and the various `"statistics_<something>"`
with equivalent `"measure_<something>"`.