TransactionRouter uses a CommitType enum to track if a commit has been started and which commit path was taken (e.g. twoPhase vs. singleShard). This enum could be extended to track when a transaction has initiated abort as well, which would simplify some of its logic, especially around retries of commit/abort and handling statements after receiving a commit/abort. This could also replace the TerminationCause enum used for logging slow transactions on mongos.