cgroups: introduce timer slack subsystem

Provides a way of tasks grouping by timer slack value. Introduces per
cgroup max and min timer slack value. When a task attaches to a cgroup,
its timer slack value adjusts (if needed) to fit min-max range.

It also provides a way to set timer slack value for all tasks in the
cgroup at once.

This functionality is useful in mobile devices where certain background
apps are attached to a cgroup and minimum wakeups are desired.