Improve the JSON PMDA by use of indom caches.
Improve the JSON PMDA by use of indom caches.

* src/pmdas/json/pmdajson.python (IndomCache): Add saving and loading the
  indom cache.
  (JsonSource): Remember the previously used IDs for clusters, metrics,
  and indoms using indom caches.

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