- added close() method to Transaction. closes out a transaction using rollback

- added close() method to Transaction.  closes out a transaction using rollback
  if it's the outermost transaction, otherwise just ends without affecting
  the outer transaction.

- transactional and non-transactional Session integrates better with bound
  connection; a close() will ensure that connection transactional state is
  the same as that which existed on it before being bound to the Session.