Improve safety of ongoing operation tracker

Finishing an operation started via
`OngoingOperationTracker.BeginOperation()` was risky in cases where the
operation ended at a callback on another thread (which, in the case of
multiplayer, is *most* cases). In particular, if any consumer registered
a callback that mutates transforms when the operation ends, it would
result in crashes after the framework-side safety checks.

Rework `OngoingOperationTracker` into an always-present component
residing in the drawable hierarchy, and ensure that the
`operationInProgress` bindable is always updated on the update thread.
This way consumers don't have to add local schedules in multiple places.

osu.Game/Tests/Visual/Multiplayer/TestMultiplayerRoomContainer.cs

@@ -25,6 +25,9 @@ namespace osu.Game.Tests.Visual.Multiplayer
         [Cached]
         public readonly Bindable<FilterCriteria> Filter = new Bindable<FilterCriteria>(new FilterCriteria());
 
+        [Cached]
+        public readonly OngoingOperationTracker OngoingOperationTracker;
+
         public TestMultiplayerRoomContainer()
         {
             RelativeSizeAxes = Axes.Both;
@@ -33,6 +36,7 @@ namespace osu.Game.Tests.Visual.Multiplayer
             {
                 Client = new TestMultiplayerClient(),
                 RoomManager = new TestMultiplayerRoomManager(),
+                OngoingOperationTracker = new OngoingOperationTracker(),
                 content = new Container { RelativeSizeAxes = Axes.Both }
             });
         }