MINOR: task: maintain a per-thread indicator of the peak run-queue size

MINOR: task: maintain a per-thread indicator of the peak run-queue size

The new field th_ctx->rq_tot_peak contains the computed peak run queue
length averaged over the last 512 calls. This is computed when entering
process_runnable_tasks. It will not take into account new tasks that are
created or woken up during this round nor those which are evicted, which
is the reason why we're using a peak measurement to increase chances to
observe transient high values. Tests have shown that 512 samples are good
to provide a relatively smooth average measurement while still fading
away in a matter of milliseconds at high loads. Since this value is
only updated once per round, it cannot be used as a statistic and
shouldn't be exposed, it's only for internal use (self-regulation).