Fixes empty recycle bin performance with indexing

Currently when the recycle bin is empty, it is going to individually delete each item from the index. This is going to cause tons of allocations in Umbraco for DeferedDeleteIndex objects for each item and then down within Examine is going to process each one individually instead of just doing it in bulk. There will be a lot of allocations made there too along with a bunch of extra and unecessary threads.

src/Umbraco.Infrastructure/Search/IndexingNotificationHandler.Content.cs

@@ -44,13 +44,21 @@ namespace Umbraco.Cms.Infrastructure.Search
                 throw new NotSupportedException();
             }
 
+            // Used to track permanent deletions so we can bulk delete from the index
+            // when needed. For example, when emptying the recycle bin, else it will
+            // individually update the index which will be much slower.
+            HashSet<int> deleteBatch = null;
+
             foreach (var payload in (ContentCacheRefresher.JsonPayload[])args.MessageObject)
             {
                 if (payload.ChangeTypes.HasType(TreeChangeTypes.Remove))
                 {
-                    // delete content entirely (with descendants)
-                    //  false: remove entirely from all indexes
-                    _umbracoIndexingHandler.DeleteIndexForEntity(payload.Id, false);
+                    if (deleteBatch == null)
+                    {
+                        deleteBatch = new HashSet<int>();
+                    }
+
+                    deleteBatch.Add(payload.Id);
                 }
                 else if (payload.ChangeTypes.HasType(TreeChangeTypes.RefreshAll))
                 {
@@ -62,6 +70,15 @@ namespace Umbraco.Cms.Infrastructure.Search
                 }
                 else // RefreshNode or RefreshBranch (maybe trashed)
                 {
+                    if (deleteBatch != null && deleteBatch.Contains(payload.Id))
+                    {
+                        // the same node has already been deleted, to ensure ordering is
+                        // handled, we'll need to execute all queued deleted items now
+                        // and reset the deleted items list.
+                        _umbracoIndexingHandler.DeleteIndexForEntities(deleteBatch, false);
+                        deleteBatch = null;
+                    }
+
                     // don't try to be too clever - refresh entirely
                     // there has to be race conditions in there ;-(
 
@@ -132,6 +149,12 @@ namespace Umbraco.Cms.Infrastructure.Search
                 //
                 // BUT ... pretty sure it is! see test "Index_Delete_Index_Item_Ensure_Heirarchy_Removed"
             }
+
+            if (deleteBatch != null)
+            {
+                // process the delete batch
+                _umbracoIndexingHandler.DeleteIndexForEntities(deleteBatch, false);
+            }
         }
     }
 }