Add option to disable the Resolver tests with an environment variable.

The resolver tests depend on external network resources; the twisted
resolver will time out when the network is unvailable instead of
returning an immediate failure like the other implementations.
Add body_producer argument to httpclient.HTTPRequest.

This allows for sending non-contiguous or asynchronously-produced
request bodies, including chunked encoding when the content-length
is not known in advance.
Add a 'partial' flag to IOStream.read_bytes.

This is a more coroutine-friendly alternative to streaming_callback.
Add a chunk_size limitation to HTTP1Connection.

HTTPMessageDelegate will no longer receive more than chunk_size bytes
in a single call.
Implement flow control for streaming RequestHandlers.