Callback based padding from application

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.
Fix broken session_detect_idle_stream()
Fix premature header block is not treated as connection error
Merge branch 'master' into draft-10
Fix compile error and test failures
Fix incoming CONTINUATION payload length is not considered in callback
Merge branch 'master' into draft-10

Conflicts:
	lib/nghttp2_session.c
Don't change state in inbound_frame_handle_pad
Add inbound_frame_reset_left() not to forget to reset iframe->buflen
Remove unused iframe->error_code