[6.x] Fix error response codes sticking to later pages in long-lived processes

[petemolinero]

When rendering an HTTP error page, RendersHttpExceptions::contents() registers a Cascade::hydrated() callback to set response_code. The problem is, that response code is currently sticky in long-lived processes (e.g. Octane). For example, after a single 404, every later page rendered by that process reports the error code in its cascade.

How we prevent response codes sticking between requests

Add a new method to handle situations where we want to register a callback that can only be consumed once, but is then removed. It's similar to flash messages in regular PHP applications ("after consumed once, delete"). Then, we use that new method for the response code callback.