Add getKey to IDBObjectStore

[inexorabletash]

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

• You can probe a single key with get(key) but pay the cost to transport/deserialize the value and don't know which key in a range was found
• You can check a range with count(query), but also don't find out the exact key
• You can do openCursor(query) and simply not continue the cursor, with minor overhead cost