feat(server): keep-alive!

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]