If we'd check it, we'd have to check it on every request. That's entirely not
worth it as I'm not even aware of any server that doesn't support psgi.streaming
right now, let alone a server people actually use.
If people complain, documentation suggesting to load the BufferedWrite
middleware needs to be written.
### Nice to have
- * make sure we're running under a server that support psgi.streaming - maybe
- just load the BufferedWrite middleware, although that might break things
- relying on ->write doing an unbuffered write
* throw away the restarter and allow using the restarters Plack provides
* remove per-request state from the engine instance
* be smarter about how we use PSGI - not every response needs to be delayed