Remove unnecessary out-of-band NuGet packages from Tsavorite.core

[PaulusParssinen]

These package, similarly to the #33, seem left overs from the FASTER implementation which targeted .NET Standard. We don't need the OOB packages such as System.Memory anymore.

Moved System.Interactive.Async reference to the Tsavorite.devices.AzureStorageDevice project which needs it for IAsyncEnumerable<TSource>.FirstAsync extension helper.

It may be worth it to use Director.Build.props to unify various shared .csproj settings into one place.