lisa.analysis.idle: Simplify and speedup df_cluster_idle_state_residency()
lisa.analysis.idle: Simplify and speedup df_cluster_idle_state_residency()

Rewrite df_cluster_idle_state_residency() to use DataFrame.groupby().
This makes the code simpler, and allows runtime under 200ms for ~80000 lines in
cpu_idle dataframe.

credp/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa