Allow the delay in the feature flag refresh loop to be cancelled (#306)

Progress on NuGet/NuGetGallery#7439

Author: joelverhagen

src/NuGet.Services.FeatureFlags/FeatureFlagCacheService.cs

@@ -88,7 +88,15 @@ public async Task RefreshAsync()
                     staleness,
                     _options.RefreshInterval);
 
-                await Task.Delay(_options.RefreshInterval);
+                try
+                {
+                    await Task.Delay(_options.RefreshInterval, cancellationToken);
+                }
+                catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+                {
+                    // Swallow the cancelled delay operation to allow quiet shutdown of the refresh loop. A cancelled
+                    // delay operation is harmless to the system.
+                }
             }
         }