[IndexedDB] Allow cancelling a versionchange event (#52)

The current model only allows for "old" connections to ignore the event (i.e. do nothing) or close the connection.

It would be nice to allow "old" connections to notify that they are not only going to ignore the request but that the "new" connection should give up, i.e. "cancel" the request.

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/52