[IndexedDB] Distinguish object store from thing IDBObjectStore represents (#3)

The spec conflates the notion of an __object store__, the persistent entity in which values are stored, with the entity that `IDBObjectStore` provides script access to, which is associated with a __transaction__.

I'm not sure what to call the latter - an __object store proxy__ ?

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https://github.com/w3c/IndexedDB/issues/3