Allow LastModified to be (internally) updated

Previously, `LastModified` was treated as readonly, to align with the original expectation that `AttributeValue` would be immutable. That's not how it's ended up being treated, however, and here we have a scenario where the `LastModified` needs to be updated. This is because `EnforceBusinessLogic` will trigger `SetValue()` indirectly via an `[AttributeSetter]` property. That's fine for updating the value. But for load and import scenarios, the `[AttributeSetter]` property will orphan the `Version` date intended to be used for the `LastModified` property. As such, we're exposing a setter to allow that property to be updated after this occurs. Since this is an outside scenario intended only to support infrastructure, however, the setter is marked as `internal`; there is no supported scenario where this should be updated directly by external callers.

Author: JeremyCaney

OnTopic/Attributes/AttributeValue.cs

@@ -189,7 +189,7 @@ internal AttributeValue(
     /// <summary>
     ///   Read-only reference to the last DateTime the <see cref="AttributeValue"/> instance was updated.
     /// </summary>
-    public DateTime LastModified { get; } = DateTime.Now;
+    public DateTime LastModified { get; internal set; } = DateTime.Now;
 
   } //Class
 } //Namespace

OnTopic/Collections/AttributeValueCollection.cs

@@ -414,6 +414,7 @@ private bool EnforceBusinessLogic(AttributeValue originalAttribute, out Attribut
         }
         _typeCache.SetPropertyValue(_associatedTopic, originalAttribute.Key, originalAttribute.Value);
         this[originalAttribute.Key].IsDirty = originalAttribute.IsDirty;
+        this[originalAttribute.Key].LastModified = originalAttribute.LastModified;
         _setCounter = 0;
         return false;
       }