Implement compliant pacing scheme for Amazon Cloud Drive

* Implement switchable pacing algorithm
  * Add tests for pacer

amazonclouddrive/amazonclouddrive.go

@@ -40,8 +40,6 @@ const (
 	statusAvailable    = "AVAILABLE"
 	timeFormat         = time.RFC3339 // 2014-03-07T22:31:12.173Z
 	minSleep           = 20 * time.Millisecond
-	maxSleep           = 15 * time.Second
-	decayConstant      = 2 // bigger for slower decay, exponential
 )
 
 // Globals
@@ -129,6 +127,7 @@ var retryErrorCodes = []int{
 	429, // Rate exceeded.
 	500, // Get occasional 500 Internal Server Error
 	409, // Conflict - happens in the unit tests a lot
+	503, // Service Unavailable
 }
 
 // shouldRetry returns a boolean as to whether this resp and err
@@ -172,7 +171,7 @@ func NewFs(name, root string) (fs.Fs, error) {
 		name:  name,
 		root:  root,
 		c:     c,
-		pacer: pacer.New().SetMinSleep(minSleep).SetMaxSleep(maxSleep).SetDecayConstant(decayConstant),
+		pacer: pacer.New().SetMinSleep(minSleep).SetPacer(pacer.AmazonCloudDrivePacer),
 	}
 
 	// Update endpoints