Use IO::Handle streams

[kjetilk]

There is code to use HTTP Conditional Requests, specifically ETag, in the underlying libraries. However, the current code has to run exactly the same logic to verify the request as it would to generate the response. All we're saving is bandwidth, no CPU.

By using IO::Handle streams on the Plack side of this module, we could also save some CPU.

At present, not all serializers support this, but they are likely to in the future.