provide last value to TxnLocal afterRollback and afterCompletion

[nbronson]

TxnLocal's afterCommit handler provides the user with the last contained value. afterRollback and afterCompletion should probably do the same thing. Currently you can work around this by explicitly registering a Txn.afterCompletion handler inside TxnLocal.initialValue, but the API should make this easier.

This will require a change to the API. Not sure if there is a clean backward compatible way.