Static code analysis - check report and fix issues

By going through static code analysis report of changes done during last a half year there are few issues that would be fine to be adjusted.

They are following

1. at com.arjuna.ats.internal.jta.transaction.arjunacore.jca.XATerminatorImple,
   L614; com.arjuna.ats.internal.jta.transaction.jts.jca, L471
   (https://github.com/jbosstm/narayana/blob/master/ArjunaJTA/jta/classes/com/arjuna/ats/internal/jta/transaction/arjunacore/jca/XATerminatorImple.java#L614) the calling method getCurrentTransactionId() potentially could end with
   NullPointerException (caused by TransactionImple#getTransaction when `final BasicAction current = BasicAction.Current()` is null)

1. possible resource leak - connection was ommitted to be closed
   1. org.jboss.narayana.tomcat.jta.integration.TestExecutor
   2. TestCommitMarkableResourceFailAfterCommitOrphan

1. inner class SampleLockable of org.jboss.stm.STMVerticle could be static (http://station5.brq.redhat.com:8080/docs/en/findbugs/fb_checker_ref.html#FB.SIC_INNER_SHOULD_BE_STATIC)