Fix cache saving stale data

After fixing a problem with object caches not being correctly invalidated we discovered a regression. Saving instance properties would trigger the object cache system to increment the timestamp on the instance and save it a second time. Unfortunately, it was also saving old field values, overwriting the changes.

The solution is to fetch the object from the database before incrementing the counter, ensuring that we do not write back stale values.
According to the Django docs, you can avoid a DB call here and avoid race conditions where two workers are trying to increment the timestamp at the same time by doing the following (https://docs.djangoproject.com/en/dev/topics/db/queries/#filters-can-reference-fields-on-the-model):

`Instance.objects.filter(pk=instance.id).update(adjuncts = F('adjuncts') + 1)