Replace one query per one email with just one queyr for all mails

Wino.Core.Domain/Interfaces/IThreadingStrategy.cs

@@ -7,6 +7,11 @@ namespace Wino.Core.Domain.Interfaces
 {
     public interface IThreadingStrategy
     {
+        /// <summary>
+        /// Attach thread mails to the list.
+        /// </summary>
+        /// <param name="items">Original mails.</param>
+        /// <returns>Original mails with thread mails.</returns>
         Task<List<IMailItem>> ThreadItemsAsync(List<MailCopy> items);
         bool ShouldThreadWithItem(IMailItem originalItem, IMailItem targetItem);
     }