SERVER-39680 introduced a startTimestamp field. We need to add upgrade/downgrade support similar to SERVER-36498 since lower versions won't recognize it and 4.2 won't have it on upgrade.
Upgrade/downgrade for oldest active transaction timestamp in transactions table
Maintain the oldest active transaction timestamp only with the transaction table
Remove 'config.transactions' entries with 'state' field on shutdown in fCV 4.0
Complete TODO listed in SERVER-40013