Process timers set for exactly now

This fixes a bug wherein the first timer set for exactly `now` ends up
in the `pending` list instead of the `ready` list, eventually resulting
in polling with a timeout of 0.

Under normal circumstances this bug would trigger very rarely, and when
it did would only result in one spurious "loop", since the next time the
timers are checked, time will have advanced.

However, this bug can cause misbehavior and deadlock in emulated
environments. e.g., in the Shadow emulator, time only moves forward when
a blocking syscall is performed, so this bug causes deadlock:
https://gitlab.torproject.org/tpo/core/arti/-/issues/174#note_2762399

Author: sporksmith

src/reactor.rs

@@ -187,7 +187,10 @@ impl Reactor {
         let now = Instant::now();
 
         // Split timers into ready and pending timers.
-        let pending = timers.split_off(&(now, 0));
+        //
+        // Careful to split just *after* `now`, so that a timer set for exactly `now` is considered
+        // ready.
+        let pending = timers.split_off(&(now + Duration::from_nanos(1), 0));
         let ready = mem::replace(&mut *timers, pending);
 
         // Calculate the duration until the next event.