MINOR: checks: use a nanosecond counters instead of timeval for checks->start

Now we store the checks start date as a nanosecond timestamps instead
of a timeval, this will simplify the operations with "now" in the near
future.
diff --git a/include/haproxy/check-t.h b/include/haproxy/check-t.h
index c489ae3..5b80189 100644
--- a/include/haproxy/check-t.h
+++ b/include/haproxy/check-t.h
@@ -156,7 +156,7 @@
 	struct buffer bi, bo;			/* input and output buffers to send/recv check */
 	struct buffer_wait buf_wait;            /* Wait list for buffer allocation */
 	struct task *task;			/* the task associated to the health check processing, NULL if disabled */
-	struct timeval start;			/* last health check start time */
+	ullong start;				/* last health check start time */
 	long duration;				/* time in ms took to finish last health check */
 	short status, code;			/* check result, check code */
 	unsigned short port;			/* the port to use for the health checks */
diff --git a/src/check.c b/src/check.c
index a8aeb9a..1d67dfd 100644
--- a/src/check.c
+++ b/src/check.c
@@ -471,7 +471,7 @@
 	if (status == HCHK_STATUS_START) {
 		check->result = CHK_RES_UNKNOWN;	/* no result yet */
 		check->desc[0] = '\0';
-		check->start = now;
+		check->start = tv_to_ns(&now);
 		return;
 	}
 
@@ -490,10 +490,10 @@
 
 	if (status == HCHK_STATUS_HANA)
 		check->duration = -1;
-	else if (!tv_iszero(&check->start)) {
+	else if (check->start) {
 		/* set_server_check_status() may be called more than once */
-		check->duration = ns_to_ms(tv_to_ns(&now) - tv_to_ns(&check->start));
-		tv_zero(&check->start);
+		check->duration = ns_to_ms(tv_to_ns(&now) - check->start);
+		check->start = 0;
 	}
 
 	/* no change is expected if no state change occurred */
@@ -1499,7 +1499,7 @@
 
 	/* check this every ms */
 	t->expire = tick_add(now_ms, MS_TO_TICKS(mininter * srvpos / nbcheck));
-	check->start = now;
+	check->start = tv_to_ns(&now);
 	task_queue(t);
 
 	return 1;
diff --git a/src/mailers.c b/src/mailers.c
index 05d5313..89732d5 100644
--- a/src/mailers.c
+++ b/src/mailers.c
@@ -145,7 +145,7 @@
 
 		/* check this in one ms */
 		t->expire    = TICK_ETERNITY;
-		check->start = now;
+		check->start = tv_to_ns(&now);
 		task_queue(t);
 	}