Filestorage: Reuse IDBObjectStore throughout transaction
Filestorage: Reuse IDBObjectStore throughout transaction

- More like passing the same reference to it when calling db_get/set.
- In reality, it was still the same store object being accessed, but
this commit improves the coherence of the db_get/set helper methods.

copy/v86,copy/v86,copy/v86,copy/v86,copy/v86,copy/v86,copy/v86