Rework saving previous papi values when adding/removing metrics
Rework saving previous papi values when adding/removing metrics

Previously, when we added or removed a metric that we were
tracking, we'd lose the previous values and all metrics would
restart from zero.  By adding a prev_metric value in papi_info,
we can systematically save the values, and add it to the atom->ull
in the fetchcallback

adfernandes/pcp,adfernandes/pcp,tjanez/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,edwardt/pcp,wuliming/pcp,edwardt/pcp,mbaldessari/pcp,tjanez/pcp,wuliming/pcp,prasincs/pcp,adfernandes/pcp,mbaldessari/pcp,mbaldessari/pcp,aeg-aeg/pcpfans,adfernandes/pcp,edwardt/pcp,wuliming/pcp,edwardt/pcp,wuliming/pcp,andyvand/cygpcpfans,prasincs/pcp,tjanez/pcp,aeg-aeg/pcpfans,tjanez/pcp,adfernandes/pcp,aeg-aeg/pcpfans,mbaldessari/pcp,prasincs/pcp,andyvand/cygpcpfans,prasincs/pcp,mbaldessari/pcp,aeg-aeg/pcpfans,tjanez/pcp,wuliming/pcp,prasincs/pcp,adfernandes/pcp,mbaldessari/pcp,wuliming/pcp,andyvand/cygpcpfans,andyvand/cygpcpfans,tjanez/pcp,andyvand/cygpcpfans,edwardt/pcp,wuliming/pcp,wuliming/pcp,edwardt/pcp,adfernandes/pcp,aeg-aeg/pcpfans,edwardt/pcp,aeg-aeg/pcpfans,andyvand/cygpcpfans,tjanez/pcp,tjanez/pcp,prasincs/pcp,edwardt/pcp,adfernandes/pcp,aeg-aeg/pcpfans,prasincs/pcp,prasincs/pcp,andyvand/cygpcpfans