Simplifying and improve build speed.

- Decreases builds from 8-13 minutes down to ~2 minutes (while running more tests)
- Moves to SQL Server 2019 on AppVeyor
- Moves the 2 longest running tests with params to `[FactLongRunning]`
- Bumps tests up to `netcoreapp3.1`
- Also fixes .Contrib tests
- Moves to `Build.csproj`
- Simplifies `build.ps1`
- Removed defunct `build.sh`
- Nukes `IsAppveyor` (moves to environmental variable overloads)
- Builds now work in Linux/macOS