nghttpx: Submit RST_STREAM if HEADERS appears in the middle of a stream
Just ignore HEADERS with non-idle stream ID and not found in stream map
Issue connection error if client receives HEADERS with idle stream ID

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.
Cleanup previous commit
doc: Add language attribute in asio_http2.h.rst.in
Robust GOAWAY handling

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.
Return error if invalid stream_id is given to nghttp2_submit_push_promise
Rename NGHTTP2_GOAWAY_FAIL_ON_SEND with NGHTTP2_GOAWAY_TERM_ON_FAIL
Rewrite session_is_new_peer_stream_id()
Influence last_stream_id to nghttp2_session_want_{read,write}
Remove unused nghttp2_stream.blocked_sent
Make sure that head_stream is not NULL
libevent-client: Disable SSLv3
Update tutorial
Reformat source code in libnghttp2_asio.rst
Add note about pseudo header ordering validation in on_header_callback
asio: Listen to all resolved addresses
nghttpx: Remove cipher suite requirement

This makes the library h2-16 compatible now.
Update README.rst
Bump up version number to 0.6.7, LT revision to 7:2:2