These options are analogous to fall and rise parameter found in
haproxy.

Merge branch 'dev/expect-continue' of https://github.com/jchampio/nghttp2 into jchampio-dev/expect-continue

Fix a crash on disconnect if --expect-continue isn't actually in use.

Each Request now owns its own (optional) ContinueTimer for
Expect/Continue handshakes. This removes the need for
shared_ptr/weak_ptr logic.

If the stream itself is reset, or the server sends a final response
immediately, any Expect/Continue handshake should be cancelled.

The ContinueTimer could expire before the full HEADERS frame was
actually sent. By moving the call to timer->start() to the
on_frame_send_callback(), this race is fixed.