added handling of Arbit log events


ndb/examples/mgmapi_logevent_example/mgmapi_logevent.cpp:
  updated logevent example
ndb/include/mgmapi/ndb_logevent.h:
  added comments
  added handling af aritration events
ndb/src/mgmapi/ndb_logevent.cpp:
  added handling of Arit log events