Read cpu.stat regardless if controller enabled. #348
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The unified hierarchy provides the cpu.stat file for every cgroup, regardless if the CPU controller is enabled (in fact, setting the systemd property CPUAccounting=True does not enable this controller because of this fact). It provides the usage_usec, user_usec, and system_usec by default. Instead of reading the stat for each enabled controller (CPU and memory), just attempt to read them each time the Stat() function is called.
Attempting to read the memory.stat file even if memory accounting is not enabled seems insignificant (some other files always have a read attempt, such as memory.current), and eliminates finding and looping over enabled controllers.
resolves #347