Netstandard2.0

[Oshi41]

netstandart 2.0 support
Code style by Resharper

• small fixes and improvements

Known issues:

• Unix ports doesn't work (tests are failed)
• Socket options are not supported (not sure if it's even supported by net standard 2.0)

public static void SetupSocket(this Socket socket, int keepAliveTime, int keepAliveInterval, int keepAliveRetryCount)
    {
        // TODO implement for net standard 2.0
    }

[Oshi41]

It's better to create a dedicated branch, I guess