apply dotnet core

[netpyoung]

• https://docs.unity3d.com/2019.3/Documentation/Manual/dotnetProfileSupport.html
• closes .NET Core 2.0 지원 #6
• fix compile error

// before
                        CPacket msg = new CPacket(buffer.Value);
// after
			CPacket msg = new CPacket(buffer.Value, null);