BUG/MINOR: mworker: fix uptime for master process

Uptime calculation for master process was incorrect as it used
<start_date> as its timestamp base time. Fix this by using the scheduler
time <start_time> for this.

The impact of this bug is minor as timestamp base time is only used for
"show proc" CLI output. it was highlighted by the following commit.
which caused a negative value to be displayed for the master process
uptime on "show proc" output.

  28360dc53f715c497fff822523f92769d8b1627d
  MEDIUM: clock: force internal time to wrap early after boot

This should be backported up to 2.0.
diff --git a/src/haproxy.c b/src/haproxy.c
index f7e0b34..507a4aa 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -2144,7 +2144,7 @@
 			}
 			tmproc->options |= PROC_O_TYPE_MASTER; /* master */
 			tmproc->pid = pid;
-			tmproc->timestamp = start_date.tv_sec;
+			tmproc->timestamp = start_time.tv_sec;
 			proc_self = tmproc;
 
 			LIST_APPEND(&proc_list, &tmproc->list);