Prevent double firing of event in consolesession

There is a timing scenario where an event could be double fired.
Prevent by clearing the event when sent.

confluent_server/confluent/consoleserver.py

@@ -552,6 +552,7 @@ class ConsoleSession(object):
         self.databuffer.append(data)
         if self._evt:
             self._evt.send()
+            self._evt = None
 
     def get_next_output(self, timeout=45):
         """Poll for next available output on this console.