Change session_start callback to async in most examples

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

examples/admin_commands.py

@@ -30,7 +30,7 @@ class AdminCommands(slixmpp.ClientXMPP):
 
         self.add_event_handler("session_start", self.start)
 
-    def start(self, event):
+    async def start(self, event):
         """
         Process the session_start event.
 
@@ -44,7 +44,7 @@ class AdminCommands(slixmpp.ClientXMPP):
                      data.
         """
         self.send_presence()
-        self.get_roster()
+        await self.get_roster()
 
         def command_success(iq, session):
             print('Command completed')