[w3c/IndexedDB] Extract common steps of `IDBObjectStore`'s `put()` and`add()` (#261)

Currently the 12 steps are almost identical, the only difference being (as far as I can tell) the value of the *no-overwrite* flag passed to [store a record into an object store](https://w3c.github.io/IndexedDB/#store-a-record-into-an-object-store). Extracting them into a separate named procedure would make it easier for users and implementers to understand the difference between the two methods.

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https://github.com/w3c/IndexedDB/issues/261