The `TransactionReaper` handler classes are not exception-safe

The TransactionReaper handler classes invariant that their finalize() method has been called, but nothing in that method's logic really requires it, nor does the TransactionReaper ensure that in any way.

Because of this, the invariants should be removed.

It is worth leaving the _finalized variables around to invariant that finalize() is only called once, but it doesn't need to be an AtomicBool since it is only used from a single thread.