Fix ColumnInfoCollection leak

Use HashSet<string> instead of ColumnInfoCollection

Author: SergeiPavlov

Orm/Xtensive.Orm/Orm/Building/Builders/IndexBuilder.cs

@@ -790,20 +790,19 @@ private static IEnumerable<TypeInfo> GatherDescendants(TypeInfo type, ICollectio
     private IEnumerable<ColumnInfo> GatherValueColumns(IEnumerable<ColumnInfo> columns)
     {
       var nameBuilder = context.NameBuilder;
-      var valueColumns = new ColumnInfoCollection(null, "ValueColumns");
+      var valueColumns = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
       foreach (var column in columns)  {
-        if (valueColumns.Contains(column.Name)) {
-          if (column.IsSystem)
-            continue;
+        if (valueColumns.Add(column.Name)) {
+          yield return column;
+        }
+        else if (!column.IsSystem) {
           var clone = column.Clone();
           clone.Name = nameBuilder.BuildColumnName(column);
           clone.Field.MappingName = clone.Name;
-          valueColumns.Add(clone);
+          _ = valueColumns.Add(clone.Name);
+          yield return clone;
         }
-        else
-          valueColumns.Add(column);
       }
-      return valueColumns;
     }
 
     private ColumnGroup BuildColumnGroup(IndexInfo index)

Orm/Xtensive.Orm/Orm/Model/ColumnInfoCollection.cs

@@ -50,9 +50,8 @@ public ICollection<ColumnInfo> Find(ColumnAttributes criteria, MatchType matchTy
 
     /// <inheritdoc/>
     public ColumnInfoCollection(Node owner, string name)
-      : base(owner, name)
+      : base(owner, name, new Dictionary<string, ColumnInfo>(StringComparer.OrdinalIgnoreCase))
     {
-      NameIndex = new Dictionary<string, ColumnInfo>(StringComparer.OrdinalIgnoreCase);
     }
   }
 }

Orm/Xtensive.Orm/Orm/Model/NodeCollection.cs

@@ -26,7 +26,7 @@ public class NodeCollection<TNode> : CollectionBaseSlim<TNode>
     [NonSerialized, DebuggerBrowsable(DebuggerBrowsableState.Never)]
     private EventHandler<ChangeNotifierEventArgs> itemChangingHandler;
 
-    protected Dictionary<string, TNode> NameIndex = new Dictionary<string, TNode>();
+    protected readonly Dictionary<string, TNode> NameIndex;
 
     /// <summary>
     /// Gets empty collection.
@@ -221,8 +221,14 @@ public override void Lock(bool recursive)
     /// <param name="owner">The owner.</param>
     /// <param name="name">The name.</param>
     public NodeCollection(Node owner, string name)
+      : this(owner, name, new Dictionary<string, TNode>())
+    {
+    }
+
+    protected NodeCollection(Node owner, string name, Dictionary<string, TNode> nameIndex)
     {
       ArgumentValidator.EnsureArgumentNotNullOrEmpty(name, "name");
+      NameIndex = nameIndex;
       Owner = owner;
       Name = name;
       itemChangingHandler = OnItemChanging;