Handle overflow in initial window update in stream

Rename window_size in nghttp2_stream as remote_window_size.
Support increment/reduction of local window size by WINDOW_UPDATE
Allow disabling auto WINDOW_UPDATE for connection and stream individually

Now NGHTTP2_OPT_NO_AUTO_WINDOW_UPDATE is split into 2 options:
NGHTTP2_OPT_NO_AUTO_STREAM_WINDOW_UPDATE and
NGHTTP2_OPT_NO_AUTO_CONNECTION_WINDOW_UPDATE.
This is preparation for the upcoming removal of END_FLOW_CONTROL
flag. For nghttpx, instead of using END_FLOW_CONTROL to disable
connection-level flow control, increase window size by large
enough value, which is friendly way to current chromium
implementation.
Rename window_size member of nghttp2_session as remote_window_size
Code cleanup