Add a way to change listening channels and unlisten to channels after Listen() is called.

It would be beneficial if Handle() and also a UnHandle() function were possible after Listen() has been called.

Start by reading the existing Listen() and Handle() entry points to understand how channels and handlers are currently registered. Determine the expected behavior for adding handlers and removing them after Listen() starts, including concurrent use, then verify that both operations work without disrupting existing listeners.

Written by the indexing model from the issue text.