Extract loading and saving certificates; certificate caching improvements

src/Titanium.Web.Proxy/Network/CachedCertificate.cs

@@ -1,23 +1,24 @@
 using System;
 using System.Security.Cryptography.X509Certificates;
+using System.Threading.Tasks;
 
 namespace Titanium.Web.Proxy.Network
 {
     /// <summary>
     ///     An object that holds the cached certificate
     /// </summary>
-    internal class CachedCertificate
+    internal sealed class CachedCertificate
     {
-        internal CachedCertificate()
-        {
-            LastAccess = DateTime.Now;
-        }
-
         internal X509Certificate2 Certificate { get; set; }
 
         /// <summary>
-        ///     last time this certificate was used
-        ///     Usefull in determining its cache lifetime
+        ///     Certificate creation task.
+        /// </summary>
+        internal Task<X509Certificate2> CreationTask { get; set; }
+
+        /// <summary>
+        ///     Last time this certificate was used.
+        ///     Useful in determining its cache lifetime.
         /// </summary>
         internal DateTime LastAccess { get; set; }
     }