Add RetrySchedule::reset()

Previously the code would do stuff like

```
    schedule = RetrySchedule::new(INITIAL_DELAY);
```

which is needlessly verbose, since the schedule already keeps track
of its initial delay.

crates/tor-basic-utils/src/retry.rs

@@ -103,6 +103,11 @@ impl RetryDelay {
     pub fn next_delay<R: Rng>(&mut self, rng: &mut R) -> Duration {
         Duration::from_millis(u64::from(self.next_delay_msec(rng)))
     }
+
+    /// Return this [`RetryDelay`] to its original state.
+    pub fn reset(&mut self) {
+        self.last_delay_ms = 0;
+    }
 }
 
 impl Default for RetryDelay {

doc/semver_status.md

@@ -18,6 +18,10 @@ We can delete older sections here after we bump the releases.
 
 ## Since Arti 0.2.0
 
+### tor-basic-util
+
+MODIFIED: Added `reset()` method to RetrySchedule.
+
 ### tor-socksproto
 
 BREAKING: Removed some unused accessors.
@@ -25,4 +29,3 @@ BREAKING: Removed some unused accessors.
 ### tor-rtmock
 
 MODIFIED: Added add_blackhole to MockNetwork.
-