bgpd: use recent monotonic time for readtime

The readtime value is for diagnostic, and doesn't have to be highly
accurate. This also fixes a problem where the readtime was being measured
with system clock, but the peer_uptime() was comparing with bgp_clock.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/bgpd/bgp_packet.c b/bgpd/bgp_packet.c
index 968c8e4..02863d7 100644
--- a/bgpd/bgp_packet.c
+++ b/bgpd/bgp_packet.c
@@ -2385,6 +2385,15 @@
   return 1;
 }
 
+/* Recent thread time.
+   On same clock base as bgp_clock (MONOTONIC)
+   but can be time of last context switch to bgp_read thread. */
+static time_t
+bgp_recent_clock (void)
+{
+  return recent_relative_time().tv_sec;
+}
+
 /* Starting point of packet process function. */
 int
 bgp_read (struct thread *thread)
@@ -2513,14 +2522,14 @@
       bgp_open_receive (peer, size); /* XXX return value ignored! */
       break;
     case BGP_MSG_UPDATE:
-      peer->readtime = time(NULL);    /* Last read timer reset */
+      peer->readtime = bgp_recent_clock ();
       bgp_update_receive (peer, size);
       break;
     case BGP_MSG_NOTIFY:
       bgp_notify_receive (peer, size);
       break;
     case BGP_MSG_KEEPALIVE:
-      peer->readtime = time(NULL);    /* Last read timer reset */
+      peer->readtime = bgp_recent_clock ();
       bgp_keepalive_receive (peer, size);
       break;
     case BGP_MSG_ROUTE_REFRESH_NEW: