Fix performance problem in JSON PMDA by reducing the number of JSON data loads.
Fix performance problem in JSON PMDA by reducing the number of JSON data loads.

* src/pmdas/json/pmdajson.python (JsonSource.fetch): If needed, reload the
  JSON data for this source.
  (JsonPMDA.__fetch): New function.
  (JsonPMDA.__refresh_metrics): Only load JSON metadata/data for new
  sources.

wuliming/pcp,prasincs/pcp,tjanez/pcp,adfernandes/pcp,tjanez/pcp,adfernandes/pcp,wuliming/pcp,aeg-aeg/pcpfans,adfernandes/pcp,prasincs/pcp,edwardt/pcp,tjanez/pcp,adfernandes/pcp,adfernandes/pcp,edwardt/pcp,prasincs/pcp,aeg-aeg/pcpfans,edwardt/pcp,wuliming/pcp,adfernandes/pcp,edwardt/pcp,wuliming/pcp,wuliming/pcp,aeg-aeg/pcpfans,wuliming/pcp,adfernandes/pcp,prasincs/pcp,edwardt/pcp,wuliming/pcp,tjanez/pcp,aeg-aeg/pcpfans,edwardt/pcp,adfernandes/pcp,prasincs/pcp,prasincs/pcp,aeg-aeg/pcpfans,prasincs/pcp,tjanez/pcp,edwardt/pcp,tjanez/pcp,aeg-aeg/pcpfans,tjanez/pcp,prasincs/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,tjanez/pcp,wuliming/pcp,edwardt/pcp