This allows to detect and handle dying streams without getting stuck
forever.
Timeouts are always wrong, though, so we put the burden of choosing the
right values (mostly) on the creator of a stream.
Without the early return in XmlStream::poll_next in case of the stream
footer, the read state gets recreated and the logic at the top of that
function to actually handle stream shutdown gracefully is never
triggered.
Also that logic was incorrect; the correct behaviour is to wait for the
true EOF.
