mod_sasl2: Further break up success handling, into pre/post stream:features This allows us to enable mod_smacks after sending success, but before sending stream:features. Previously, the features were being calculated before SM was finalized on the session. We can't finalize SM before sending <success> because it may generate stanzas.

This module adds a command to `prosodyctl` for listing users.

``` sh
prosodyctl mod_listusers
```