tweak NTP parameters

- allow up to 3 step corrections for non-boundary servers that are one
  second or more out of sync on startup.
- limit boundary server polling to 8 seconds

These changes solve two problems that are regularly observed in a4x2

1. Non-boundary servers make corrections at infinitesimal rates, and can
   take hours to converge.
2. Boundary servers can take several minutes to show up in chronyc
   tracking due to polling intervals of a minute or more.

Fixes #5192

zone-setup/src/bin/zone-setup.rs

@@ -296,16 +296,13 @@ logdir /var/log/chrony
 log measurements statistics tracking
 
 # makestep <threshold> <limit>
-# We allow chrony to step the system clock if we are more than a day out,
-# regardless of how many clock updates have occurred since boot.
-# The boundary NTP servers are configured with local reference mode, which
-# means that if they start up without external connectivity, they will appear
-# as authoritative servers even if they are advertising January 1987
-# (which is the default system clock on a gimlet after boot).
-# This configuration allows a one-off adjustment once RSS begins and the
-# boundary servers are synchronised, after which the clock will advance
-# monotonically forwards.
-makestep 86400 -1
+# We allow chrony to step the system clock up to three times on startup if we
+# are more than a second out. Even though boundary NTP servers are configured
+# with local reference mode, they will not appear as authoritative servers
+# until they synchronize upstream. This configuration allows us to converge
+# quickly once the boundary servers converge. After this the clock will advance
+# monotonically forward.
+makestep 1.0 3
 
 # When a leap second occurs we slew the clock over approximately 37 seconds.
 leapsecmode slew
@@ -369,8 +366,10 @@ maxslewrate 2708.333
 
     let new_config = if *is_boundary {
         for s in servers {
-            let str_line =
-                format!("pool {} iburst maxdelay 0.1 maxsources 16\n", s);
+            let str_line = format!(
+                "pool {} iburst maxdelay 0.1 minpoll 0 maxpoll 3 maxsources 16\n",
+                s
+            );
             contents.push_str(&str_line)
         }
         contents