Remove stream feature handlers on session_start.

Based on profiling, using around 35 stream handlers quarters the number
of basic message stanzas that can be processed in a second, in
comparison to only using the bare minimum of four handlers.

To help, we can drop handlers for stream features once the session
has started. So that we can re-enable these handlers when a stream
must restart, the 'stream_start' event has been added which fires
whenever a stream header is received.

The 'stream_start' event is a more generic replacement for the
existing start_stream_handler() method.

sleekxmpp/test/sleektest.py

@@ -367,6 +367,7 @@ 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)