nghttpx: Minor tweak to buffer size

Simplified when to send WINDOW_UPDATE to the backend, that is we send
WINDOW_UPDATE when input buffer is empty.
Remove useless debug code in nghttp2_stream_roots_add()
Take into account that pending_local_max_concurrent_stream could be too large

pending_local_max_concurrent_stream is, once local settings applied,
becomes NGHTTP2_INITIAL_MAX_CONCURRENT_STREAMS, which is very large
number.  When adjusting number of streams, we have to take min of
local effective SETTINGS_MAX_CONCURRENT_STREAMS and pending one.