Fixed IDisposable implementation and usage.

- Implement IDisposable interface correctly, with sealed classes where possible for simplicity.
- Add using statement around undisposed local variables.

OpenRA.Game/Network/ReplayConnection.cs

@@ -16,7 +16,7 @@ using OpenRA.Primitives;
 
 namespace OpenRA.Network
 {
-	public class ReplayConnection : IConnection
+	public sealed class ReplayConnection : IConnection
 	{
 		class Chunk
 		{