MEDIUM: task: remove the tasks_run_queue counter and have one per thread

MEDIUM: task: remove the tasks_run_queue counter and have one per thread

This counter is solely used for reporting in the stats and is the hottest
thread contention point to date. Moving it to the scheduler and having a
separate one for the global run queue dramatically improves the performance,
showing a 12% boost on the request rate on 16 threads!

In addition, the thread debugging output which used to rely on rqueue_size
was not totally accurate as it would only report task counts. Now we can
return the exact thread's run queue length.

It is also interesting to note that there are still a few other task/tasklet
counters in the scheduler that are not efficiently updated because some cover
a single area and others cover multiple areas. It looks like having a distinct
counter for each of the following entries would help and would keep the code
a bit cleaner:
  - global run queue (tree)
  - per-thread run queue (tree)
  - per-thread shared tasklets list
  - per-thread local lists

Maybe even splitting the shared tasklets lists between pure tasklets and
tasks instead of having the whole and tasks would simplify the code because
there remain a number of places where several counters have to be updated.

(cherry picked from commit 9c7b8085f4cad284642e7f67d2274f2fb568f243)
[wt: backported as it does have a significant impact on many-core machines;
 with 48 threads, the request rate is simply doubled and the response time
 halved; There were significant context updates, all verified to be OK; it
 does not seem reasonable to backport it to 2.2]
Signed-off-by: Willy Tarreau <w@1wt.eu>