Fix delayed server shutdown if clients are connected to the Watch server-streaming RPC of the Health API

[freibenjamin]

Fixes #2572

Changes

• Copied the Grpc.HealthCheck.HealthServiceImpl class to the Grpc.AspNetCore.HealthChecks namespace. This was necessary because the HealthServiceImpl now requires the IHostApplicationLifetime, which is not available in the Grpc.HealthCheck package. I assumed you prefer not to modify references of that package. I did not change the original implementation to ensure backwards compatibility.
• Used the ApplicationStopping cancellation token in the Grpc.AspNetCore.HealthChecks.HealthServiceImpl class to gracefully terminate Watch streams during server shutdown. The ApplicationStopping token was passed to the WaitToReadAsync method to allow running calls to finish when the server is stopping. In the catch block, a NotServing response is written to the response stream, though an RpcException could be thrown if preferred.
• Minor changes to comments.

Testing

The following screenshot shows that the server now stops within a second, even if clients are connected to the Watch RPC at the time of shutdown. The server was stopped at 12:09:17 (via the terminal).

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[JamesNK]

Thanks for the PR. This is a good improvement but I don't like duplicating the implementation of the health checks service.

I've made a new PR that adds the same capability but with a different implementaiton: #2582

[freibenjamin]

Thank you, @JamesNK, for addressing the issue. I agree that duplicating the health check service wasn't the most elegant approach.