Add analyzers for .NET Core 3.0

[davidfowl]

These are raw meeting notes that will be cleaned up later by @shirhatti

Useful vs-threading analyzers for async usage

VS Threading has a bunch of useful analyzers that we'd like to push into the various parts of the core SDK. Here's the summary of things we decided about each of them.

https://github.com/Microsoft/vs-threading/blob/master/doc/analyzers/index.md#diagnostic-analyzers

• VSTHRD001 - not interesting without JTF
• VSTHRD002 - analyzer very interesting everywhere. Code Fix may fail, so maybe not interesting.
• VSTHRD003 - not interesting without JTF
• VSTHRD004 - not interesting without JTF
• VSTHRD010 - interesting to UI framework, has code fix
• VSTHRD011 - interesting AFTER asynclazy is in the framework.
• VSTHRD012 - not interesting without JTF
• VSTHRD100 - interesting for Core
• VSTHRD101 - interesting for Core
• VSTHRD102 - no. too nuanced
• VSTHRD103 - interesting for Core
• VSTHRD104 - interesting for Core (2nd tier)
• VSTHRD105 - Core tier 2
• VSTHRD106 - No
• VSTHRD107 - interesting to Core
• ??? - catch returning a Task from a method within a using block, without awaiting
• VSTHRD108 - Interesting for UI
• VSTHRD109 - Yes for UI stack
• VSTHRD110 - Yes for Core
• VSTHRD111 - Yes for Core
• VSTHRD200 - Yes for Core

Analyzers for API usage

• HttpClient usage
• Don't await Task.Run in ASP.NET Core context
• Methods you should avoid in an async context
• It'll be data driven e.g. Monitor.Enter, BlockingCollection.Add*
• Block Task.Factory.StartNew and ContinueWith
• Always create TaskCompletionSource with TaskCreationOptions.RunContinuationsAsynchronously
• Never use TaskContinuationOptions.RunContinuationsAsynchronously in tcs ctor
• Always dispose CancellationTokenSource(s)
• Always flow CancellationToken(s) to APIs that take a CancellationToken
• Prefer async using vs using in and async method if the type has IAsyncDisposable
• Prefer async/await over directly returning Task (maybe)

ASP.NET Core

• Do not cache the access to IHttpContextAccessor.HttpContext
• Do not capture services injected into the controllers on background threads