[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>

src/Discord.Net.Rest/Entities/Messages/RestMessage.cs

@@ -1,9 +1,13 @@
+using Discord.API;
+
 using Newtonsoft.Json.Linq;
+
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
 using System.Threading.Tasks;
+
 using Model = Discord.API.Message;
 
 namespace Discord.Rest
@@ -45,6 +49,12 @@ namespace Discord.Rest
         /// <inheritdoc />
         public virtual bool MentionedEveryone => false;
 
+        /// <inheritdoc cref="IMessage.Thread"/>
+        public RestThreadChannel Thread { get; private set; }
+
+        /// <inheritdoc />
+        IThreadChannel IMessage.Thread => Thread;
+
         /// <summary>
         ///     Gets a collection of the <see cref="Attachment"/>'s on the message.
         /// </summary>
@@ -80,15 +90,6 @@ namespace Discord.Rest
         /// <inheritdoc/>
         public MessageType Type { get; private set; }
 
-
-        /// <summary>
-        ///     Gets the thread that was started from this message.
-        /// </summary>
-        /// <returns>
-        ///    A <see cref="RestThreadChannel"/> object if this message has thread attached; otherwise <see langword="null"/>.
-        /// </returns>
-        public RestThreadChannel Thread { get; private set; }
-
         /// <inheritdoc />
         public MessageRoleSubscriptionData RoleSubscriptionData { get; private set; }
 
@@ -266,9 +267,7 @@ namespace Discord.Rest
             }
 
             if (model.Thread.IsSpecified)
-            {
                 Thread = RestThreadChannel.Create(Discord, new RestGuild(Discord, model.Thread.Value.GuildId.Value), model.Thread.Value);
-            }
         }
         /// <inheritdoc />
         public async Task UpdateAsync(RequestOptions options = null)
@@ -310,7 +309,7 @@ namespace Discord.Rest
 
         /// <inheritdoc />
         IReadOnlyCollection<IStickerItem> IMessage.Stickers => Stickers;
-        
+
         #endregion
 
         /// <inheritdoc />

src/Discord.Net.WebSocket/Entities/Messages/SocketMessage.cs

@@ -81,6 +81,12 @@ namespace Discord.WebSocket
         /// <inheritdoc />
         public MessageRoleSubscriptionData RoleSubscriptionData { get; private set; }
 
+        /// <inheritdoc cref="IMessage.Thread"/>
+        public SocketThreadChannel Thread { get; private set; }
+
+        /// <inheritdoc />
+        IThreadChannel IMessage.Thread => Thread;
+
         /// <summary>
         ///     Returns all attachments included in this message.
         /// </summary>
@@ -283,6 +289,12 @@ namespace Discord.WebSocket
                     model.RoleSubscriptionData.Value.MonthsSubscribed,
                     model.RoleSubscriptionData.Value.IsRenewal);
             }
+
+            if (model.Thread.IsSpecified)
+            {
+                SocketGuild guild = (Channel as SocketGuildChannel)?.Guild;
+                Thread = guild?.AddOrUpdateChannel(state, model.Thread.Value) as SocketThreadChannel;
+            }
         }
 
         /// <inheritdoc />