sync: use custom bson serializer for SyncItem for perf

Notesnook.API/Startup.cs

@@ -159,6 +159,7 @@ namespace Notesnook.API
                 options.CacheDuration = TimeSpan.FromMinutes(30);
             });
 
+            BsonSerializer.RegisterSerializer(new SyncItemBsonSerializer());
             if (!BsonClassMap.IsClassMapRegistered(typeof(UserSettings)))
                 BsonClassMap.RegisterClassMap<UserSettings>();