Revert "Remove stream feature handlers on session_start."

This reverts commit 4274f49ada.

The SASL mech was choking on this, so let's send it back for some
more refining.

sleekxmpp/test/sleektest.py

@@ -367,7 +367,6 @@ class SleekTest(unittest.TestCase):
         if skip:
             if socket != 'live':
                 # Mark send queue as usable
-                self.xmpp.event('session_start')
                 self.xmpp.session_started_event.set()
                 # Clear startup stanzas
                 self.xmpp.socket.next_sent(timeout=1)