Fix Repeater memoization memory leak

If the ConditionalWeakTable references itself, it will get leaked by
.NET Core GC: dotnet/runtime#12255

Fortunately, we don't really need the ConditinalWeakTable here:
 * Repeater should generally be a short-lived object, so
       strong references will also be collected reasonably soon
 * Between Load and PreRender, the viewmodels were pinned
       in the DataContext of all children and the children are in Children
       collection. Only after PreRender could those references be useful,
       but we can as well Clear the dictionary at that point.

    Co-authored-by: Adam Štěpánek <[email protected]>

src/Framework/Framework/Controls/Repeater.cs

@@ -9,6 +9,8 @@
 using System.Diagnostics;
 using System.Linq;
 using System.Runtime.CompilerServices;
+using System.Collections.Generic;
+using DotVVM.Framework.Utils;
 
 namespace DotVVM.Framework.Controls
 {
@@ -138,6 +140,7 @@ protected internal override void OnLoad(IDotvvmRequestContext context)
         protected internal override void OnPreRender(IDotvvmRequestContext context)
         {
             SetChildren(context, renderClientTemplate: !RenderOnServer, memoizeReferences: false);
+            childrenCache.Clear(); // not going to need the unreferenced children anymore
             base.OnPreRender(context);
         }
 
@@ -246,7 +249,7 @@ private DotvvmControl GetEmptyItem(IDotvvmRequestContext context)
             return emptyDataContainer;
         }
 
-        private ConditionalWeakTable<object, DataItemContainer> childrenCache = new ConditionalWeakTable<object, DataItemContainer>();
+        private readonly Dictionary<object, DataItemContainer> childrenCache = new(ReferenceEqualityComparer<object>.Instance);
         private DotvvmControl GetItem(IDotvvmRequestContext context, object? item = null, int? index = null, bool allowMemoizationRetrieve = false, bool allowMemoizationStore = false)
         {
             if (allowMemoizationRetrieve && item != null && childrenCache.TryGetValue(item, out var container2) && container2.Parent == null)
@@ -272,8 +275,12 @@ private DotvvmControl GetItem(IDotvvmRequestContext context, object? item = null
             // write it to the cache after the content is build. If it would be before that, exception could be suppressed
             if (allowMemoizationStore && item != null)
             {
-                // this GetValue call adds the value without raising exception when the value is already added...
-                childrenCache.GetValue(item, _ => container);
+#if DotNetCore
+                childrenCache.TryAdd(item, container);
+#else
+                if (!childrenCache.ContainsKey(item))
+                    childrenCache.Add(item, container);
+#endif
             }
 
             return container;

src/Framework/Framework/Utils/ReferenceEqualityComparer.cs

@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+
+namespace DotVVM.Framework.Utils
+{
+    // TODO next version: Replace with System.Collections.Generic.ReferenceEqualityComparer
+    internal class ReferenceEqualityComparer<T> : IEqualityComparer<T>
+        where T : class
+    {
+        public static ReferenceEqualityComparer<T> Instance { get; } = new ReferenceEqualityComparer<T>();
+
+
+        public bool Equals(T? x, T? y) => ReferenceEquals(x, y);
+        public int GetHashCode(T obj) => obj == null ? 0 : RuntimeHelpers.GetHashCode(obj);
+    }
+}