consolidate Infrastructure interface and implementation classes

Oqtane.Server/Infrastructure/ServerStateManager.cs

@@ -4,6 +4,11 @@ using System.Linq;
 namespace Oqtane.Infrastructure
 {
     // singleton
+    public interface IServerStateManager
+    {
+        ServerState GetServerState(string siteKey);
+    }
+
     public class ServerStateManager : IServerStateManager
     {
         private List<ServerState> _serverStates { get; set; }