WebSocket upgrade has an issue, the "websocket upgrade" is actually happening during the "accept" call currently.
This is bad, because it blocks the accept loop while we are upgrading a peer

Ideally, we would tweak nim-websock to split the "accept" & "upgrade" part, and then only call accept in accept, and upgrade in upgrade

Will also enable us to improve TransportOsError handling cc @Ivansete-status