This commit introduces a major rewrite of go-watchdog.
* HeapDriven and SystemDriven are now distinct run modes.
* WIP ProcessDriven that uses cgroups.
* Policies are now stateless, pure and greatly simplified.
* Policies now return the next utilization at which GC
should run. The watchdog enforces that value differently
depending on the run mode.
* The heap-driven run mode adjusts GOGC dynamically. This
places the responsibility on the Go runtime to honour the
trigger point, and results in more robust logic that is not
vulnerable to very quick bursts within sampling periods.
* The heap-driven run mode is no longer polling (interval-driven).
Instead, it relies entirely on GC signals.
* The Silence and Emergency features of the watermark policy
have been removed. If utilization is above the last watermark,
the policy will request immediate GC.
* Races removed.
