[IndexedDB] Add getKey to IDBObjectStore (#26)

For performance, as well as symmetry with `IDBIndex`, add `getKey()` to `IDBObjectStore`

* You can probe a single key with `get(key)` but not with a more complex query (e.g. range)
* You can check a range with `count(query)`, but don't find out the exact key and it must iterate the range
* You can do `openCursor(query)` and simply not continue the cursor, with minor overhead cost


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Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/26