Fixing various TransactionXAResource bugs
 - eliminated 'newTxn' and reused 'transaction' local variable instead to reduce the count of used local method variables
   (there was also a bug in the code related to these two local variables where we used
   'transaction' instead of 'newTxn' in transactionUpdater.compareAndSet() method calls)
 - setting ok flag to 'true' to indicate transaction association succeded without errors
eliminating useless do-while from TransactionXAResource.end() method
TransactionXAResource.rollback() maps to transaction.rollback()
fixing TransactionXAResource serialization issue
fixing javadoc warnings