Use the new .NET Standard 2.1 ADO.NET async APIs everywhere

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Some missing ADO.NET async APIs have been added to .NET Core 3.0, and will hopefully make it into .NET Standard 2.1 soon. When that happens we need to go over our code and make sure we're calling the ADO.NET async versions from our own async methods.

Note: this should include using IAsyncDisposable.