Merge branch 'master' into multiplayer-beatmap-tracker

osu.Game/Online/Rooms/BeatmapAvailability.cs

@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using MessagePack;
 using Newtonsoft.Json;
 
 namespace osu.Game.Online.Rooms
@@ -9,11 +10,13 @@ namespace osu.Game.Online.Rooms
     /// <summary>
     /// The local availability information about a certain beatmap for the client.
     /// </summary>
+    [MessagePackObject]
     public class BeatmapAvailability : IEquatable<BeatmapAvailability>
     {
         /// <summary>
         /// The beatmap's availability state.
         /// </summary>
+        [Key(0)]
         public readonly DownloadState State;
 
         /// <summary>