This change will utilize last_stream_id in GOAWAY extensively.  When
GOAWAY is received with a last_stream_id, library closes all outgoing
streams whose stream_id > received last_stream_id.
nghttp2_on_stream_callback is called for each stream to be closed.

When GOAWAY is sent with a last_stream_id, library closes all incoming
streams whose stream_id > sent last_stream_id.
nghttp2_on_stream_callback is called for each stream to be closed.

If stream ID is not idle, it might be valid HEADERS.  If stream ID is
idle, it is invalid regardless stream ID is even or odd, since client
is not expected to recieve request from server.  nghttp2 library
historically allows this, but now we forbids this.

nghttp2_session.c:636:3: warning: Branch condition evaluates to a garbage value

Merge branch 'Wunused-parameter' of https://github.com/alagoutte/nghttp2 into alagoutte-Wunused-parameter

turotial -> tutorial

We make following HEADERS under priority control:
* push response HEADERS
* HEADERS submitted by nghttp2_submit_response

Currently, HEADERS submitted by nghttp2_submit_headers is not attached
to stream.  This is because it may be used as non-final response
header and application may submit final response using
nghttp2_submit_response without checking non-final response header
transmission.