Fix Memory (event handlers) leak of ColumnInfoCollection objects

Author: SergeiPavlov

Orm/Xtensive.Orm/Orm/Model/FieldInfo.cs

@@ -1,4 +1,4 @@
-// Copyright (C) 2007-2021 Xtensive LLC.
+// Copyright (C) 2007-2022 Xtensive LLC.
 // This code is distributed under MIT license terms.
 // See the License.txt file in the project root for more information.
 // Created by: Dmitri Maximov
@@ -627,6 +627,7 @@ public ColumnInfoCollection Columns
 
         var result = new ColumnInfoCollection(this, "Columns");
         GetColumns(result);
+        columns = result;
         return result;
       }
     }