MINOR: task: introduce a thread-local "sched" variable for local scheduler stuff

The aim is to rassemble all scheduler information related to the current
thread. It simply points to task_per_thread[tid] without having to perform
the operation at each time. We save around 1.2 kB of code on performance
sensitive paths and increase the request rate by almost 1%.

src/debug.c

@@ -77,7 +77,7 @@ void ha_thread_dump(struct buffer *buf, int thr, int calling_tid)
 		return;
 
 	chunk_appendf(buf, "             curr_task=");
-	ha_task_dump(buf, curr_task, "             ");
+	ha_task_dump(buf, sched->current, "             ");
 }