We need paddings regardless of payload and frame boundary to mitigate
certain attacks.

Since we handles CONTINUATION internally, we don't show FLAG_PAD_HIGH
and PAD_LOW flags of HEADERS in nghttp/nghttpd. We just show the
total paddings in HEADERS + CONTINUATION.

The only drawback is that page source only contains include
directive. We disabled page source for now.

* Use 1 Huffman code table for both request and response
* Remove complicated deflater side table size management
* Add encoding context update
* Fix memory leak in inflater

This may help the pathological situation where window is too small.

Conflicts:
	lib/nghttp2_session.c

Now previous padding options are removed and instead we added
select_padding_callback to select padding length for each frame
by application. If this callback is not implemented by application,
no padding is added.

This change also fixes the broken session_detect_idle_stream()
if stream_id is our side.