MINOR: mux-quic: use sched call time for pacing

QUIC pacing was recently implemented to limit burst and improve overall
bandwidth. This is used only for MUX STREAM emission. Pacing requires
nanosecond resolution. As such, it used now_cpu_time() which relies on
clock_gettime() syscall.

The usage of clock_gettime() as several drawbacks :
* it is a syscall and requires a context-switch
* it is not be available on all systems
* value is changing accross a single task execution which may tamper
  pacing calculation

Improve this by using task_mono_time() instead. This requires the flag
TASK_F_WANTS_TIME on QUIC MUX tasklet to require the scheduler to update
sched call time with now_mono_time(). This solves any limitations listed
above :
* syscall invokation is only performed once before tasklet execution,
  thus reducing context-switch impact
* on non compatible system, a millisecond timer is used as a fallback
  which should ensure that pacing works decently for them
* timer value is now guaranteed to be fixed duing task execution

Author: a-denoyelle

include/haproxy/mux_quic-t.h

@@ -18,6 +18,7 @@
 #include <haproxy/quic_pacing-t.h>
 #include <haproxy/quic_stream-t.h>
 #include <haproxy/stconn-t.h>
+#include <haproxy/task-t.h>
 #include <haproxy/time-t.h>
 
 /* Stream types */

src/mux_quic.c

@@ -2996,6 +2996,7 @@ static int qmux_init(struct connection *conn, struct proxy *prx,
 
 	qcc->wait_event.tasklet->process = qcc_io_cb;
 	qcc->wait_event.tasklet->context = qcc;
+	qcc->wait_event.tasklet->state  |= TASK_F_WANTS_TIME;
 	qcc->wait_event.events = 0;
 
 	qcc->proxy = prx;

src/quic_pacing.c

@@ -1,16 +1,17 @@
 #include <haproxy/quic_pacing.h>
 
 #include <haproxy/quic_tx.h>
+#include <haproxy/task.h>
 
 /* Returns true if <pacer> timer is expired and emission can be retried. */
 int quic_pacing_expired(const struct quic_pacer *pacer)
 {
-	return !pacer->next || pacer->next <= now_mono_time();
+	return !pacer->next || pacer->next <= task_mono_time();
 }
 
 /* Notify <pacer> about an emission of <sent> count of datagrams. */
 void quic_pacing_sent_done(struct quic_pacer *pacer, int sent)
 {
-	pacer->next = now_mono_time() + pacer->cc->algo->pacing_rate(pacer->cc) * sent;
+	pacer->next = task_mono_time() + pacer->cc->algo->pacing_rate(pacer->cc) * sent;
 	pacer->last_sent = sent;
 }