docs: public synchronized void begin()
      {
          checkForBegin();
          if (log.isDebugEnabled()) log.debug("Begin transaction was called on tx " + this);
          // initialize the ObjectEnvelope table
          objectEnvelopeTable = new ObjectEnvelopeTable(this);
          // register transaction
          implementation.getTxManager().registerTx(this);
          // mark tx as active (open)
          txStatus = Status.STATUS_ACTIVE;
      }