Change locking mechanism for multiplayer rooms to use using-disposal pattern

Was required to lock over `await` calls server-side.

osu.Game/Online/RealtimeMultiplayer/LockUntilDisposal.cs

@@ -0,0 +1,24 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Threading;
+
+namespace osu.Game.Online.RealtimeMultiplayer
+{
+    public readonly struct LockUntilDisposal : IDisposable
+    {
+        private readonly object lockTarget;
+
+        public LockUntilDisposal(object lockTarget)
+        {
+            this.lockTarget = lockTarget;
+            Monitor.Enter(lockTarget);
+        }
+
+        public void Dispose()
+        {
+            Monitor.Exit(lockTarget);
+        }
+    }
+}

osu.Game/Online/RealtimeMultiplayer/MultiplayerRoom.cs

@@ -5,7 +5,6 @@
 
 using System;
 using System.Collections.Generic;
-using JetBrains.Annotations;
 
 namespace osu.Game.Online.RealtimeMultiplayer
 {
@@ -48,12 +47,8 @@ namespace osu.Game.Online.RealtimeMultiplayer
         }
 
         /// <summary>
-        /// Perform an update on this room in a thread-safe manner.
+        /// Request a lock on this room to perform a thread-safe update.
         /// </summary>
-        /// <param name="action">The action to perform.</param>
+        public LockUntilDisposal LockForUpdate() => new LockUntilDisposal(writeLock);
-        public void PerformUpdate([InstantHandle] Action<MultiplayerRoom> action)
-        {
-            lock (writeLock) action(this);
-        }
     }
 }