- added incrementer/decrementer for the number of IXFR/AXFR running into
   the
   XfroutCounter class

 - added getter methods of counter handlers for the XfroutSession/NotifyOut
   class into the XfroutCounter class

 - inserted incrementer/decrementer handler for the number of AXFR/IXFR
   running into XfroutSession

 - removed the unnecessary thread lock from the get_statistics()

 - removed a commented-out code

 - updated copyright