Fix leak in FieldInfo.UpdateState()

Author: SergeiPavlov

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

@@ -661,8 +661,8 @@ public override void UpdateState()
       Fields.UpdateState();
       if (column != null)
         column.UpdateState();
-      columns = new ColumnInfoCollection(this, "Columns");
-      GetColumns(columns);
+      columns?.Clear();           // To prevent event handler leak
+      columns = null;
 
       HasImmediateValidators = validators.Count > 0 && validators.Any(v => v.IsImmediate);
 

Orm/Xtensive.Orm/Orm/Model/TypeInfo.cs

@@ -748,7 +748,7 @@ private bool GetIsLeaf()
     private void CreateTupleDescriptor()
     {
       var orderedColumns = columns.OrderBy(c => c.Field.MappingInfo.Offset).ToList();
-      columns.Clear();
+      columns.Clear();                    // To prevent event handler leak
       columns.AddRange(orderedColumns);
       TupleDescriptor = TupleDescriptor.Create(
         Columns.Select(c => c.ValueType).ToArray(Columns.Count));