- added the new class XfroutCounter
  It is intended for producing incrementers of the counters based on the
  statistics spec and for outputting statistics data format to passed to the
  b10-stats daemon.
- added counter handlers passed to the NotifyOut object and the XfroutSession
  object
- added handling of the getstats command
- added counting xfr requests in the XfroutSession class
- updated the UnixSockServer class for taking over the counter handlers