support Cosmos DB .NET SDK 3.0

[felschr]

https://azure.microsoft.com/en-us/blog/azure-cosmos-dotnet-sdk-version-3-0-now-in-public-preview/

Support should be added before the stable launch. The current preview release has some fundamental features missing but I'll be looking into this matter again once it's more stable.

Due to the amount of changes I'm going to remove support for older SDKs.
Among those changes the SDK finally supports .NET Standard 2.0.

[felschr]

To match the major version of the Cosmos DB SDK I moved this to the 3.0 milestone.