Change session_start callback to async in most examples

If we fetch the roster, we should probably wait until we get it back

examples/echo_client.py

@@ -38,7 +38,7 @@ class EchoBot(slixmpp.ClientXMPP):
         # MUC messages and error messages.
         self.add_event_handler("message", self.message)
 
-    def start(self, event):
+    async def start(self, event):
         """
         Process the session_start event.
 
@@ -52,7 +52,7 @@ class EchoBot(slixmpp.ClientXMPP):
                      data.
         """
         self.send_presence()
-        self.get_roster()
+        await self.get_roster()
 
     def message(self, msg):
         """