message: Reduce mempool.cs locks by using secondary polltimer
  We don't really need to have the orphan pool size nor
  the bytes transmitted updated mutilple times per second
  as we do using polltimer1.  We can use polltimer2 which
  has a 5 second trigger and is perfectly adequate.
diff: @@ -135,13 +135,16 @@ void ClientModel::updateTimer1()
  // no locking required at this point
  // the following calls will aquire the required lock
  Q_EMIT mempoolSizeChanged(getMempoolSize(), getMempoolDynamicUsage());
  - Q_EMIT orphanPoolSizeChanged(getOrphanPoolSize());
  - Q_EMIT bytesChanged(getTotalBytesRecv(), getTotalBytesSent());
  Q_EMIT transactionsPerSecondChanged(getTransactionsPerSecond());
  }
  void ClientModel::updateTimer2()
  {
  + // no locking required at this point
  + // the following calls will aquire the required lock
  + Q_EMIT orphanPoolSizeChanged(getOrphanPoolSize());
  + Q_EMIT bytesChanged(getTotalBytesRecv(), getTotalBytesSent());
  +
  uiInterface.BannedListChanged();
  }
  