Refactoring WebSocket connections

[annevk]

In whatwg/fetch#1244 @ricea started a draft of how WebSockets could take partitioning better into account and maybe also share more logic with Fetch's connection infrastructure. That work is still important, but now needs to happen here as whatwg/fetch#1516 will remove the parts this specification took over from Fetch.

I didn't want to move whatwg/fetch#1243 over as it seems good to keep track of this issue on the Fetch side and the discussion there is no longer as relevant as a number of decisions have already been made.