rev: async response bodies with DevFdResponse middleware

rev: async response bodies with DevFdResponse middleware

This new middleware should be a no-op for non-Rev concurrency
models (or by explicitly setting env['rainbows.autochunk'] to
false).

Setting env['rainbows.autochunk'] to true (the default when Rev
is used) allows (e)poll-able IO objects (sockets, pipes) to be
sent asynchronously after app.call(env) returns.

This also has a fortunate side effect of introducing a code path
which allows large, static files to be sent without slurping
them into a Rev IO::Buffer, too.  This new change works even
without the DevFdResponse middleware, so you won't have to
reconfigure your app.

This lets us epoll on response bodies that come in from a pipe
or even a socket and send them either straight through or with
chunked encoding.