As a followup to WT-4052, it was pointed out that the transaction snapshot array can add up to some significant memory. The array is sized at the max number of sessions the system is sized for, and each element of the array is 8 bytes. MongoDB has session_max=20000, so the burden for each MongoDB session is 160K. If we could free this array on session reset, it should reduce memory footprint for idle sessions. We'd need to find a way to lazily allocate it on first use.