[core/sched/stats] Add support to trace task latency and bursts

If SCHEDSTATS are enabled, for each SE is tracked:
- exec_delay: time the SE has waited into a RQ since when it was ready
              to run;
- exec_slice: the CPU time the SE has got the last time has been scheduled
              for execution on a CPU.

A new sched_info_latency_track method has been added which is used to reset
the exec tracking counters for the next reading. Such a call is meant to be
called right after a SE execution has completed.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>
[core/sched/events] Add tracing of SE latency and slice

Each time a SE is releasing the CPU, a new "latency" event is traced which
reports:
- delay: the time a SE was waiting for a CPU since last time it was ready
         to run;
- slice: the time a SE has executed since last time has got the CPU, which
         is right now leaving;

Signed-off-by: Patrick Bellasi <derkling@gmail.com>
[core/sched/cbs] Enable, by default, REINIT_ON_YIELD

Reinitialization on Yield has been experimentally evaluated to be better,
thus we set this feature on by default for a CBS enabled kernel.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>