[REEF-1584] Fix CoreCLR Issues in Org.Apache.REEF.IO

This addressed the issue by
  * Targeting .NET Core 2.0 Framework
  * Rewriting the DirectoryInfo.Create(DirectorySecurity) and CreateSubdirectory(DirectorySecurity) commands to use the SetAccessControl APIs that are supported in .NET Core 2.0
    * See the Extension Methods section of the [.NET Standard 2.0 documentation](https://docs.microsoft.com/en-us/dotnet/api/system.io.directoryinfo?view=netstandard-2.0) for details on the supported APIs. (Note that these extensions are all .NET Core 2.0, not .NET Standard.)

JIRA:
  [REEF-1584](https://issues.apache.org/jira/browse/REEF-1584)

Pull request:
  This closes #1351