Internals have been shuffled around such that Request and Reponse are
now given only a mutable reference to the stream, instead of being
allowed to consume it. This allows the server to re-use the streams if
keep-alive is true.
A task pool is used, and the number of the threads can currently be
adjusted by using the `listen_threads()` method on Server.
[breaking-change]
A connection is returned from Incoming.next(), and can be passed to a
separate thread before any parsing happens. Call conn.open() to get a
Result<(Request, Response)>.
BREAKING CHANGE
