Replace Fleck with a more resilent solution

[Kaliumhexacyanoferrat]

As the maintainer of the web socket module, I would like to replace the Fleck 3rd-party dependency with a more robust solution, so that users do not encounter strange issues.

Fleck implementation mixes a lot of concerns and does lack coding quality. We should fork Fleck to create a more maintainable version.

We can either:

1. Move the code into GenHTTP
2. Create a new fork and make it usable for GenHTTP (basically the same way as we do currently)