Track total active time of transaction in SingleTransactionStats

We want to track the total amount of "active" and "inactive" time spent by a transaction in SingleTransactionStats. A transaction is considered "active" if it is currently executing an operation, and considered "inactive" if it is idly waiting for a new operation to occur. To measure "active" time, we can use the "unstash" and "stash" transaction events to mark the beginning and end of an "active" period. When an operation stashes its transaction resources, we should add that operation's total running time to a cumulative SingleTransactionStats::timeActive field on the session. Similarly, when the first operation on a transaction completes, we should add its running time to SingleTransactionStats::timeActive.