COMMIT needs to return mutation tokens, ATR data, and state

Part of the transaction API is TransactionResult.mutationState(), containing the final mutation tokens of all documents involved in the transaction.  This is intended to allow RYOWs post-transaction.

There are places where the ATR (which can be selected by query) and AttemptState are also needed:

• Know whether to do an implicit commit, and prevent double commit/rollback.  (Not strictly required, as I could track this with additional state, e.g. has commit/rollback been called.)
• Differentiate COMMITTED vs COMPLETED so I can set TransactionResult.unstagingComplete correctly.
• Populate the list of TransactionAttempts inside TransactionResult, which is used by the tests to e.g. verify the ATR data is written correctly.  

So it would be helped to have this information returned in the COMMIT and ROLLBACK.