Proxy blocking when reaching browser SSE stream limit

[grelland]

Hi, and thanks for a great tool!

I'm using the proxy whilst developing a basic template-based web application. It works mostly well, but I noticed that on every seventh page load the browser will hang for about a minute before the request is handled.

After some debugging I traced the problem to be the per-browser/hostname SSE stream limit (which just so happens to be exactly 6), coupled with the cleanup logic in the reloadHandler here. This blocks until the (dropped) connection is eventually cleaned up and the browser is able to establish the EventSource.

I have a fix in mind and will work on a PR 👍