Make int64 based atomic ratelimiter default (#101)

* Fix return timestamp discrepancy between regular atomic limiter and int64 based one
* Make int64 based atomic limiter default

Long story: this was added in #85, but reverted in #91 due to #90. #95 fixed the issue, so we're moving forward with the new implementation.

limiter_atomic_int64.go

@@ -82,6 +82,12 @@ func (t *atomicInt64Limiter) Take() time.Time {
 			break
 		}
 	}
-	t.clock.Sleep(time.Duration(newTimeOfNextPermissionIssue - now))
-	return time.Unix(0, newTimeOfNextPermissionIssue)
+
+	sleepDuration := time.Duration(newTimeOfNextPermissionIssue - now)
+	if sleepDuration > 0 {
+		t.clock.Sleep(sleepDuration)
+		return time.Unix(0, newTimeOfNextPermissionIssue)
+	}
+	// return now if we don't sleep as atomicLimiter does
+	return time.Unix(0, now)
 }

ratelimit.go

@@ -54,7 +54,7 @@ type config struct {
 
 // New returns a Limiter that will limit to the given RPS.
 func New(rate int, opts ...Option) Limiter {
-	return newAtomicBased(rate, opts...)
+	return newAtomicInt64Based(rate, opts...)
 }
 
 // buildConfig combines defaults with options.