optimize plex tv scanner (#190)

ErsatzTV.Core/Plex/PlexTelevisionLibraryScanner.cs

@@ -70,7 +70,17 @@ namespace ErsatzTV.Core.Plex
                         await maybeShow.Match(
                             async result =>
                             {
-                                await ScanSeasons(library, result.Item, connection, token);
+                                if (result.IsAdded || incoming.ShowMetadata.Head().DateUpdated >
+                                    result.Item.ShowMetadata.Head().DateUpdated)
+                                {
+                                    await ScanSeasons(library, result.Item, connection, token);
+                                }
+                                else
+                                {
+                                    _logger.LogDebug(
+                                        "Skipping Plex show that has not been updated: {Show}",
+                                        incoming.ShowMetadata.Head().Title);
+                                }
 
                                 if (result.IsAdded)
                                 {