TransactionReaper should use something other than lastWriteOpTimeTs for staleness

The TransactionReaper mistakenly uses the time component of lastWriteOpTimeTs to notice transaction records over N minutes old (which we use to decide when transaction records are eligible for deletion due to a lack of a session record).

We should flush an additional field to the transaction table which includes a wall clock time to compare against, as well as a redirecting the reaper to use that field for eligibility.