* Initial envoy cgroup resource monitor implementation
* Add cgroupv1 support; add metrics instrumentation
* Slight refactor for more efficient memory limit detection
Instead of reading memory.max/limit_in_bytes on every tick, we
read it once, then again only when it is modified.
To support this change, logic for computing the saturation was moved out
of the cgroup driver and into the resource monitor, and the driver
interface now has separate methods for reading memory usage and limit.
* Code cleanup/lint fixes
* Add platform build tags
* Add unit tests
* Fix lint issues
* Add runtime flag to allow disabling resource monitor
* Clamp saturation values to the range [0.0, 1.0]
* Switch to x/sys/unix; handle inotify IN_IGNORED events
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