Change the Handler trait to receive an Iterator of (Request, Response) pairs.

This allows downstream users to have total control of their concurrency
strategy, while also exposing a very nice, streaming interface for frameworks
to build on.

This also resolves issues surrounding the use of IoResult as the return type
of Handler::handle, because handlers now have complete control over how to
handle internal failure.

Fixes #3
Fixes #4
Why consume the handler here?