The kCommittingWithPrepare and kCommittingWithoutPrepare transaction states were originally added because needed to release the TransactionParticipant mutex before calling into the OpObserver, and a concurrent operation could modify the TransactionParticipant state while the mutex was not held. After SERVER-38810, however, the session must be checked out in order to modify TransactionParticipant state, so the kCommittingWithPrepare and kCommittingWithoutPrepare transaction states are likely unnecessary.