nghttpx: Add upgrade-scheme parameter to backend option

If "upgrade-scheme" parameter is present in backend option along with
"tls" paramter, HTTP/2 :scheme pseudo header field is changed to
"https" from "http" when forwarding a request to this particular
backend.  This is a workaround for a server which requests "https"
scheme on HTTP/2 connection encrypted by TLS.

Allow PING frame to be sent after GOAWAY

nghttpx: Remember which resource is pushed

nghttpx: Fix missing ALPN validation (--npn-list)

This commit fixes the bug that ALPN validation does not occur when
ALPN list is not sent from client.

Remember which resource is pushed in order to conform to the semantics
described in RFC 8297.

src: Define 103 status code

Remove SPDY

This reverts commit f507b5ee.

Balancing load is more important at the moment.

Fix CMAKE_MODULE_PATH

nghttpx: Write API request body in temporary file

nghttpx: Increase api-max-request-body

nghttpx: Faster configuration loading with lots of backends

nghttpx: Fix crash with --backend-http-proxy-uri option

Use NGHTTP2_REFUSED_STREAM for streams which are closed by GOAWAY

The error code NGHTTP2_REFUSED_STREAM is passed to
nghttp2_on_stream_close callback when a stream is closed because its
stream ID is strictly larger than incoming or outgoing GOAWAY.

Fix doc