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/Settings.cs

@@ -91,6 +91,9 @@ namespace OpenRA
 		[Desc("For dedicated servers only, save replays for all games played.")]
 		public bool RecordReplays = false;
 
+		[Desc("For dedicated servers only, treat maps that fail the lint checks as invalid.")]
+		public bool EnableLintChecks = true;
+
 		public ServerSettings Clone()
 		{
 			return (ServerSettings)MemberwiseClone();