Bumping txnNumber on Session must always abort running transaction with lower txnNumber

If a transaction with a higher txnNumber is started, but it fails validation and uasserts in TransactionParticipant::beginOrContinue() prior to aborting the previous transaction, then the txnNumber on the Session will be the new value, but there will still be an in-progress transaction in TransactionParticipant. The in-progress transaction is no longer accessible, since the txnNumber on Session has been bumped, so a client cannot abort it. We should never bump the txnNumber on Session without aborting the previous transaction.