Adjust Windows sleep time to avoid premature wakeup.

c++/src/kj/mutex.c++

@@ -511,6 +511,11 @@ void Mutex::lockWhen(Predicate& predicate, Maybe<Duration> timeout) {
   LARGE_INTEGER endTime;
 
   KJ_IF_MAYBE(t, timeout) {
+    // Windows sleeps are inaccurate -- they can be longer *or shorter* than the requested amount.
+    // For many use cases of our API, a too-short sleep would be unacceptable. Experimentally, it
+    // seems like sleeps can be up to half a millisecond short, so we'll add a millisecond.
+    *t += 500 * kj::MICROSECONDS;
+
     // Compute initial sleep time.
     sleepMs = *t / kj::MILLISECONDS;
     if (*t % kj::MILLISECONDS > 0 * kj::SECONDS) {