MEDIUM: tasks: Get rid of active_tasks_mask.

Remove the active_tasks_mask variable, we can deduce if we've work to do
by other means, and it is costly to maintain. Instead, introduce a new
function, thread_has_tasks(), that returns non-zero if there's tasks
scheduled for the thread, zero otherwise.
diff --git a/src/debug.c b/src/debug.c
index b34df72..fd760fa 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -47,7 +47,7 @@
 	              "%c%cThread %-2u: act=%d glob=%d wq=%d rq=%d tl=%d tlsz=%d rqsz=%d\n"
 	              "             stuck=%d fdcache=%d prof=%d",
 	              (thr == calling_tid) ? '*' : ' ', stuck ? '>' : ' ', thr + 1,
-	              !!(active_tasks_mask & thr_bit),
+		      thread_has_tasks(),
 	              !!(global_tasks_mask & thr_bit),
 	              !eb_is_empty(&task_per_thread[thr].timers),
 	              !eb_is_empty(&task_per_thread[thr].rqueue),