Skip LFU rescheduling when expiry hasn't changed (#580)

* skip reschedule

* cleanup

---------

Author: bitfaster

BitFaster.Caching/Lfu/NodePolicy.cs

@@ -97,9 +97,12 @@ public void AdvanceTime()
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void OnRead(TimeOrderNode<K, V> node)
         {
-            var currentExpiry = node.TimeToExpire - current;
-            node.TimeToExpire = current + expiryCalculator.GetExpireAfterRead(node.Key, node.Value, currentExpiry);
-            wheel.Reschedule(node);
+            var oldTte = node.TimeToExpire;
+            node.TimeToExpire = current + expiryCalculator.GetExpireAfterRead(node.Key, node.Value, node.TimeToExpire - current);
+            if (oldTte.raw != node.TimeToExpire.raw)
+            {
+                wheel.Reschedule(node);
+            }
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -113,9 +116,12 @@ public void OnWrite(TimeOrderNode<K, V> node)
             }
             else
             {
-                var currentExpiry = node.TimeToExpire - current;
-                node.TimeToExpire = current + expiryCalculator.GetExpireAfterUpdate(node.Key, node.Value, currentExpiry);
-                wheel.Reschedule(node);
+                var oldTte = node.TimeToExpire;
+                node.TimeToExpire = current + expiryCalculator.GetExpireAfterUpdate(node.Key, node.Value, node.TimeToExpire - current);
+                if (oldTte.raw != node.TimeToExpire.raw)
+                {
+                    wheel.Reschedule(node);
+                }
             }
         }