For FileUploaders it's possible that a model has many uploads
and if lease key is created only from model id, it causes that
the model's uploads can not be migrated in parallel because the
exclusive lease key would be same for all uploads of the model.
This is tricky: the query was being run in
`ObjectStorage::Extension::RecordsUploads#retrieve_from_store!`, but we can't
just add batch loading there, because the `#upload=` method there would use the
result immediately, making the batch only have one item.
Instead, we can pre-emptively add an item to the batch whenever an avatarable
object is initialized, and then reuse that batch item in
`#retrieve_from_store!`. However, this also has problems:
1. There is a lot of logic in `Avatarable#retrieve_upload_from_batch`.
2. Some of that logic constructs a 'fake' model for the batch key. This should
be fine, because of ActiveRecord's override of `#==`, but it relies on that
staying the same.
Old records have a null value of file_store column.
This causes the problems with current direct_upload implementation,
as this makes it to choose Store::REMOTE instead of Store::LOCAL.
This change moves the store save when change saving the object.
