Clarifying abort behaviours

[guybedford]

It could be worth further clarifying the drop & abort behaviours at various stages of the lifecycle of a request and response concretely, since these are observable implementation details in HTTP.

Specifically -

• Should the destructor for a FutureIncomingResponse that has not resolved abort the request?
• A drop of a FutureIncomingResponse should probably also therefore also invalidate the outgoing body, so that any writes or finish calls shouldn't work.
• Should dropping an incoming-response or incoming-body or input-stream for a response also cause the connection to be immediately aborted?

And similarly for the server handler cases as well.