Release 3.1.0

[MaceWindu]

Tracks list of tasks to to:

• [x] move project to main linq2db repository and obsolete this one
• [x] update project to VS2019
• [x] update project to use C#9
• [x] update project to use NRT
• [ ] automate build/testing/deploy using azure (integrate into existing pipelines)
• [ ] address existing issues
• [ ] check if we need to update code from https://github.com/dotnet/aspnetcore/tree/master/src/Identity