The max length of header block is not used right now. It will
be used when header continuation is implemented.

To match the -w16 to 65535, which is HTTP/2.0 default initial window
size, decrement 1 from (1 << window_bits).

nghttpx does not perform upgrade if the request has request body.

The previous value 100 is non-standard, and if it is not sent with
SETTINGS, session will be closed with protocol error.

Handle HTTP Upgrade and CONNECT explicitly

Fix segmentation fault if DATA frame is received for the stream
which is already closed.