Avoid keeping H/2 streams alive indefinitely.

This adds timeouts for:
  - How long we allow a stream to wait for a WINDOW_UPDATE, subject
    to idle_send_timeout
  - Total duration after we started transmitting a response,
    subject to send_timeout

We currently do not make any attempt at sending a GOAWAY or a PING for
client inactivity. This may be something to reconsider at a later point.