Redis adapter is not correctly cleaning up/unsubscribing topics when handling socketIO middleware errors

[carera]

hello, found a bug where redis adapter is not correctly cleaning up/unsubscribing topics when handling socketIO middleware errors:

• When we call next(new Error('Foo') in a socketIO middleware (as suggested here), socketIO lib calls _cleanup on error here
• (note that, at this point, redis-adapter is already subscribed to topics in Redis)
• _cleanup implementation calls leaveAll, which is here and which calls adapter.delAll
• socketIO adapter interface defines delAll here (it calls this._del in a loop)
• redis adapter however does not override/extend this method with custom behaviour, which, i think, should clean up Redis subscriptions

This causes leaking subscriptions on Redis servers of clients that are long gone.

Added a draft of what seems to fix the problem, though not sure whether this is the most correct approach, and also I don't know how to test this 🙈.

[carera]

Realised this is not a correct approach, will discuss in socketio/socket.io#4602 (comment) first