changed Session.msg_id() property to

use session_id and message counter.
This gives a unique id but uses much less time

Author: tomoboy

jupyter_client/session.py

@@ -527,10 +527,12 @@ def clone(self):
         new_session.digest_history.update(self.digest_history)
         return new_session
 
+    message_count = 0
     @property
     def msg_id(self):
-        """always return new uuid"""
-        return new_id()
+        message_number = self.message_count
+        self.message_count += 1
+        return '{}_{}'.format(self.session, message_number)
 
     def _check_packers(self):
         """check packers for datetime support."""