Process for host instance notification shouldn't rely on file watchers

[mattchenderson]

This item is to track investigation into alternative methods for coordination across host instances, such as a queue based model, etc.

Today, file-based sentinels are used, but this has a few issues:

1. This can be missed in situations where the assumption of a shared file system isn't upheld. That can manifest as a silent failure for the scenario, though the impact would be lessened by other coordinated recycle events such as configuration changes to the resource.
2. Using file-based also creates a requirement on a mutable filesystem being present, which limits hosting environment options.
3. File watchers have a material perf impact. We should strive to move away from them.

An example scenario can be seen with the default secrets repository:

azure-functions-host/src/WebJobs.Script.WebHost/Security/KeyManagement/BlobStorageSecretsRepository.cs

Lines 140 to 141 in ed038be

	string filePath = GetSecretsSentinelFilePath(type, functionName);
	await FileUtility.WriteAsync(filePath, DateTime.UtcNow.ToString());