Fix for unhealthy host restart

[soninaren]

Issue describing the changes in this PR

With worker indexing enabled all of our worker channels are managed at the webhost level. In such a scenario, if the host becomes unhealthy we restart the script host here

azure-functions-host/src/WebJobs.Script.WebHost/WebJobsScriptHostService.cs

Line 724 in af3687d

var tIgnore = RestartHostAsync(CancellationToken.None);

However, as part of host restart we do not shutdown the workers started at the webhost level. Once the new host starts up specifically for dotnet-isolated, the request to load the same function is sent again. As the worker has not been restarted it throws an invalidOperation exception with the error. Result: Failure Exception: System.InvalidOperationException: Unable to load Function '[Function name]'. A function with the id '[Function id]' name already exists

Additional PR information

The fix here was to make sure we shutdown the workers started at the webhost level on host shutdown. Except for when we specialize.

resolves #2124

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
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Additional information

Additional PR information

[mathewc]

nit: fix camel casing of Webhost on prop and constants. The codebase uses WebHost