The metadata parser unconditionally assigned GetInt results (which
return 0 for missing keys) to all KCP config fields, overwriting
defaults like SndWnd=1024, RcvWnd=1024, StreamBuf=2097152 with 0.
KCP's WndSize only applies positive values, so snd_wnd=0 caused
the write path condition waitsnd < s.kcp.snd_wnd to never succeed,
effectively stalling throughput at ~10Mbps instead of 400Mbps+.
- Only apply metadata values when the key actually exists (IsExists check)
- Use Clone() instead of sharing the global DefaultConfig pointer
- Add Config.Clone() deep-copy method
sagitchu