Rework MapPreview custom rule handling.

The previous asynchronous approach did not work particularly well,
leading to large janks when switching to custom maps or opening the
mission browser.

This commit introduces two key changes:

 * Rule loading for WorldActorInfo and PlayerActorInfo is made
   synchronous, in preparation for the next commit which will
   significantly optimize this path.
 * The full ruleset loading, which is required for map validation,
   is moved to the server-side and managed by a new ServerMapStatusCache.
   The previous syntax check is expanded to include the ability to run
   lint tests.

OpenRA.Game/Server/Server.cs

@@ -61,6 +61,7 @@ namespace OpenRA.Server
 
 		// Managed by LobbyCommands
 		public MapPreview Map;
+		public readonly MapStatusCache MapStatusCache;
 		public GameSave GameSave = null;
 
 		readonly int randomSeed;
@@ -170,6 +171,17 @@ namespace OpenRA.Server
 			}.Serialize());
 		}
 
+		void MapStatusChanged(string uid, Session.MapStatus status)
+		{
+			lock (LobbyInfo)
+			{
+				if (LobbyInfo.GlobalSettings.Map == uid)
+					LobbyInfo.GlobalSettings.MapStatus = status;
+
+				SyncLobbyInfo();
+			}
+		}
+
 		public Server(List<IPEndPoint> endpoints, ServerSettings settings, ModData modData, ServerType type)
 		{
 			Log.AddChannel("server", "server.log", true);
@@ -229,12 +241,16 @@ namespace OpenRA.Server
 
 			serverTraits.TrimExcess();
 
+			Map = ModData.MapCache[settings.Map];
+			MapStatusCache = new MapStatusCache(modData, MapStatusChanged, type == ServerType.Dedicated && settings.EnableLintChecks);
+
 			LobbyInfo = new Session
 			{
 				GlobalSettings =
 				{
 					RandomSeed = randomSeed,
-					Map = settings.Map,
+					Map = Map.Uid,
+					MapStatus = Session.MapStatus.Unknown,
 					ServerName = settings.Name,
 					EnableSingleplayer = settings.EnableSingleplayer || Type != ServerType.Dedicated,
 					EnableSyncReports = settings.EnableSyncReports,
@@ -254,6 +270,8 @@ namespace OpenRA.Server
 
 			new Thread(_ =>
 			{
+				// Initial status is set off the main thread to avoid triggering a load screen when joining a skirmish game
+				LobbyInfo.GlobalSettings.MapStatus = MapStatusCache[Map];
 				foreach (var t in serverTraits.WithInterface<INotifyServerStart>())
 					t.ServerStarted(this);
 
@@ -541,7 +559,7 @@ namespace OpenRA.Server
 								SendOrderTo(newConn, "Message", motd);
 						}
 
-						if (Map.DefinesUnsafeCustomRules)
+						if ((LobbyInfo.GlobalSettings.MapStatus & Session.MapStatus.UnsafeCustomRules) != 0)
 							SendOrderTo(newConn, "Message", "This map contains custom rules. Game experience may change.");
 
 						if (!LobbyInfo.GlobalSettings.EnableSingleplayer)