[MINOR] http: differentiate waiting for new request and waiting for a complete requst

[MINOR] http: differentiate waiting for new request and waiting for a complete requst

While waiting in a keep-alive state for a request, we want to silently
close if we don't get anything. However if we get a partial request it's
different because that means the client has started to send something.
This requires a new transaction flag. It will be used to implement a
distinct timeout for keep-alive and requests.