Change how ACKs are checked

[keith-turner]

With the change in #645 its possible that an observer could be run for a transactions that rolled back. This would happen in the case where a transaction writers notifications and then fails to complete the 2nd phase of commit. Need to change how ACKs are checked in the prewrite iterator to how it was done before the changes in cc257fb. Before this change ACKs were checked to see if a write occurred after an ACK.