AsyncState can be used across different service providers

[mobratil]

Current implementation of AsyncState has problems.
This PR fixes #4959 by making the field _contextCount static.

Microsoft Reviewers: Open in CodeFlow

[RussKie]

Few more issues to tackle

[sebastienros]

The counter is always increasing and declared as int. Is the counter tied to the pool, meaning that it won't increase more than the number of pooled objects, or is it incremented with each AsyncState resolution (multiple times per request) and possibly exceeding the int capacity?

[mobratil]

@sebastienros Yes, the int is increasing but only when we create a new instance of AsyncContext<>. But we register these types as singletons (

extensions/src/Libraries/Microsoft.Extensions.AsyncState/AsyncStateExtensions.cs

Line 27 in 88d79f7

services.TryAddSingleton(typeof(IAsyncContext<>), typeof(AsyncContext<>));

) so there's limited number of these instances.