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 blocks configured queries that change server state.

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

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