Reconnect terminal if websocket was closed

Fixes #1844

If the websocket was closed (seeing this happen in container scenarios), mark the terminal as disconnected so the reconnection logic kicks in. For example, terminal should reconnect if the user types in the disconnected terminal.

Prior to this fix, the terminal would remain inactive and ignore input if the underlying websocket had closed.

src/gwt/src/org/rstudio/studio/client/workbench/views/terminal/TerminalSession.java

@@ -265,7 +265,13 @@ public void receivedOutput(String output)
       }
       socket_.dispatchOutput(output, doLocalEcho());
    }
-
+
+   @Override
+   public void connectionDisconnected()
+   {
+      disconnect(false);
+   }
+
    public void receivedSendToTerminal(String input)
    {
       // tweak line endings 

src/gwt/src/org/rstudio/studio/client/workbench/views/terminal/TerminalSessionSocket.java

@@ -60,6 +60,11 @@ public interface Session
        * @param output output from server
        */
       void receivedOutput(String output);
+
+      /**
+       * Called to disconnect the terminal
+       */
+      void connectionDisconnected();
    }
 
    public interface ConnectCallback
@@ -237,6 +242,7 @@ public void onClose(CloseEvent event)
             {
                diagnostic("WebSocket closed");
                socket_ = null;
+               session_.connectionDisconnected();
             }
 
             @Override