C#DK Performance & Quality via .NET Install Tool

The startup experience at launch in C# and C#DevKit, as well as other extensions that rely on the .NET Install Tool, received a large level of focus this quarter. For the median user we have saved at least 800+ ms from startup time with the existing work, but much more for the percentile of users with slower computers or networks.

This also involves the update and download experience, which can be improved from the current state of needing to check for a new .NET runtime every time that C# and C#DK are initialized. This check and update can take minutes for many users leading to a bad experience.

Investments were also made to 'async' up the codebase and properly implement a safe mutex experience across multiple windows of vscode/code insiders simultaneously while handling process kill scenarios better, which all also tied to the performance story.

Issues:
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2177
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2209
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2199
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2217
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2229
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2233
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2234
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2251
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2288
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2293
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2294
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2296
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2304
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2305
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2309
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2317
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2332
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2334
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2339

Unresolved:
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2354
- https://github.com/dotnet/vscode-dotnet-runtime/pull/2306
- https://github.com/dotnet/vscode-dotnet-runtime/issues/2314
- https://github.com/dotnet/vscode-dotnet-runtime/issues/2276
- https://github.com/dotnet/vscode-dotnet-runtime/issues/2269
- https://github.com/dotnet/vscode-dotnet-runtime/issues/2315
- https://github.com/microsoft/vscode-dotnettools/issues/2158
- https://github.com/dotnet/vscode-dotnet-runtime/issues/2359