Release 3.1.0

[MaceWindu]

Tracks list of tasks to to:

• move project to main linq2db repository and obsolete this one
• update project to VS2019
• update project to use C#9
• 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