pacer: make pacer more flexible

Make the pacer package more flexible by extracting the pace calculation
functions into a separate interface. This also allows to move features
that require the fs package like logging and custom errors into the fs
package.

Also add a RetryAfterError sentinel error that can be used to signal a
desired retry time to the Calculator.

backend/s3/s3.go

@@ -782,7 +782,7 @@ type Fs struct {
 	bucketOKMu    sync.Mutex       // mutex to protect bucket OK
 	bucketOK      bool             // true if we have created the bucket
 	bucketDeleted bool             // true if we have deleted the bucket
-	pacer         *pacer.Pacer     // To pace the API calls
+	pacer         *fs.Pacer        // To pace the API calls
 	srv           *http.Client     // a plain http client
 }
 
@@ -1055,7 +1055,7 @@ func NewFs(name, root string, m configmap.Mapper) (fs.Fs, error) {
 		c:      c,
 		bucket: bucket,
 		ses:    ses,
-		pacer:  pacer.New().SetMinSleep(minSleep).SetPacer(pacer.S3Pacer),
+		pacer:  fs.NewPacer(pacer.NewS3(pacer.MinSleep(minSleep))),
 		srv:    fshttp.NewClient(fs.Config),
 	}
 	f.features = (&fs.Features{