Deadlocked transactions

A database lock manager dumps a snapshot of who is waiting for whom. txn_ids lists every open transaction, and waits holds entries written as "t3|t7", meaning t3 is blocked on a lock that t7 holds. A transaction can be blocked on only one lock at a time, so it appears at most once on the left of an entry, every name in waits also appears in txn_ids, and the ids are distinct. A transaction waiting on nobody is running right now, and a blocked transaction resumes as soon as the transaction it waits for finishes. Return the ids of the transactions that will never resume, sorted in ascending order, or an empty list when the snapshot is healthy. Remember that a transaction queued behind a stuck one is stuck too, even when it sits outside the cycle, and that a transaction waiting on itself never resumes.