Fixes #130

The following error was raised "AttributeError: Socket has no such option: ON_RECV" errors during kernel restart operations.

Changes

Kernel Client (kernel_client.py)

• Enhanced stop_listening() method: Added proper attribute checking with hasattr() to prevent AttributeError when
  _listening_task doesn't exist
• Added cleanup in finally block: Ensures _listening_task is properly reset to None after cancellation to prevent
  stale references

Kernel Manager (kernel_manager.py)

• Improved disconnect() method: Added null check for main_client before attempting to stop listening and
  channels, preventing errors when client is already disconnected

Multi-Kernel Manager (multi_kernel_manager.py)

• Added restart_kernel() method: Implemented proper kernel restart that bypasses problematic activity watching
  and buffering, calling the pinned superclass method directly to avoid ZMQ socket issues

Problem Solved

The original error occurred because jupyter_server's _async_restart_kernel method was trying to use outdated ZMQ
socket options. This fix works around the issue by:

1. Properly handling missing attributes during client cleanup
2. Adding safety checks for null clients during disconnection
3. Implementing a clean restart path that avoids the problematic ZMQ code