feat: use rfc339 in Timestamp::standard_format()

This causes timestamps to display as:

  2024-05-03T07:40:52.287-07:00

instead of:

  Thu, 2 May 2024 12:05:43 -0700

The new style includes sub-second precision, omitting trailing zeros
which is very useful for logfiles.

src/models/consensus/timestamp.rs

@@ -105,7 +105,7 @@ impl Timestamp {
             NaiveDateTime::from_timestamp_millis(self.0.value().try_into().unwrap_or(0)).unwrap();
         let utc: DateTime<Utc> = DateTime::from_naive_utc_and_offset(naive, *Utc::now().offset());
         let offset: DateTime<Local> = DateTime::from(utc);
-        offset.to_rfc2822()
+        offset.to_rfc3339_opts(chrono::SecondsFormat::AutoSi, false)
     }
 
     pub fn arbitrary_between(start: Timestamp, stop: Timestamp) -> BoxedStrategy<Timestamp> {