These started to appear when I `^C` `curl -N http://localhost:5052/beacon/fork/stream`: `Aug 12 13:00:01.539 ERRO Couldn't stream piece hyper::Error(ChannelClosed), service: http`
Something must have changed in hyper since SSE has been implemented because I'm sure I haven't seen those errors before.
This PR properly detects a closed SSE stream and cleans up.