[IndexedDB] Ensure connection/handle properties are snapshots (#20)

Clean up the normative text around various connection/handle properties so it is clear these do not change after the connection closes/transaction ends:

- [*] IDBDatabase's objectStoreNames
- [ ] IDBDatabase's version
- [*] IDBObjectStore's indexNames


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Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/20