Snapshot isolation commit

Under snapshot isolation with first-committer-wins, a transaction must ABORT at commit time if any item it wrote was also written by a transaction that committed during its lifetime (between its start and its commit). Transactions are given as rows [start_ts, commit_ts, write_items] sorted by commit_ts ascending (all commit_ts distinct). Process them in commit order; a transaction commits successfully only if none of its write_items conflict with the write_items of an already-committed transaction whose commit_ts is strictly greater than this transaction's start_ts. Aborted transactions do NOT apply their writes (don't count against later transactions). Return the list of start_ts values of the transactions that successfully COMMIT, in commit order.