[Feature] Rework Thread property of a message (#2579)

* move `Thread` property to `IUserMessage` & add one to `SocketUserMessage`

* get channel from cache instead of creating new object

* move the property back to IMessage

src/Discord.Net.Core/Entities/Messages/IMessage.cs

@@ -89,6 +89,14 @@ namespace Discord
         /// </summary>
         IUser Author { get; }
 
+        /// <summary>
+        ///     Gets the thread that was started from this message.
+        /// </summary>
+        /// <returns>
+        ///    An <see cref="IThreadChannel"/> object if this message has thread attached; otherwise <see langword="null"/>.
+        /// </returns>
+        IThreadChannel Thread { get; }
+
         /// <summary>
         ///     Gets all attachments included in this message.
         /// </summary>