Support body write interruption

If a peer stops writing the body of a request or response, a Hyper client or server has the ability to detect that and handle it however it likes. However, the same is not true if the peer stops reading the body of the request or response that Hyper is sending to it - the write will just indefinitely hang waiting on space in the stream.

There should be some way for consumers to tell Hyper to abort the write. For example, there could be a poll_abort method on Body which Hyper will poll even when it can't request another body chunk. The body implementation could maintain whatever timeout logic it wants internally and ensure the task is woken up at the appropriate time.

Start by reviewing Hyper's Body API and the request/response write path to understand where a consumer can signal that a peer has stopped reading. The issue is done when consumers have a defined way to abort a blocked body write and the behavior is covered by the relevant tests.