Have shell sessions not autoreconnect

Unlike consoles, where the underlying concept is a real
persistent thing that needs some care to reattach to watch,
a shell session should die when it is lost, as a new one would
have to be created anyway.  Modify the disconnect behavior
for a shell session to set closed and notify the receivers.
It should also reap dependent watching objects in a future
change.

confluent_server/confluent/shellserver.py

@@ -34,6 +34,10 @@ class _ShellHandler(consoleserver.ConsoleHandler):
         # suppress logging through proving a stub 'log' function
         return
 
+    def _got_disconnected(self):
+        self.connectstate = 'closed'
+        self._send_rcpts({'connectstate': self.connectstate})
+