Updates petapoco to have a sliding expiration for the delegates cache, updates the keys it uses to be short hashes, fixes some null checks in content/media services, adds new test for checking that peta poco caches aren't growing with the same queries.

src/Umbraco.Core/Services/ContentService.cs

@@ -523,6 +523,10 @@ namespace Umbraco.Core.Services
         public IEnumerable<IContent> GetDescendants(int id)
         {
             var content = GetById(id);
+            if (content == null)
+            {
+                return Enumerable.Empty<IContent>();
+            }
             return GetDescendants(content);
         }