Slight change to the way we offer XMPP stream features.

Slight change to the way we offer XMPP stream features.
Before authentication, the only feature we offer is SASL, implying to
the client that the only thing it is allowed to do is log in.  After
a successful SASL authentication, which causes the XMPP stream to be
restarted, the features we offer are sessions and binding, but not
SASL.  This allows clients to initialize in the correct sequence and
guarantees that we will have an email address to use in the JID.