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/box/box.go

@@ -111,7 +111,7 @@ type Fs struct {
 	features     *fs.Features          // optional features
 	srv          *rest.Client          // the connection to the one drive server
 	dirCache     *dircache.DirCache    // Map of directory path to directory id
-	pacer        *pacer.Pacer          // pacer for API calls
+	pacer        *fs.Pacer             // pacer for API calls
 	tokenRenewer *oauthutil.Renew      // renew the token on expiry
 	uploadToken  *pacer.TokenDispenser // control concurrency
 }
@@ -260,7 +260,7 @@ func NewFs(name, root string, m configmap.Mapper) (fs.Fs, error) {
 		root:        root,
 		opt:         *opt,
 		srv:         rest.NewClient(oAuthClient).SetRoot(rootURL),
-		pacer:       pacer.New().SetMinSleep(minSleep).SetMaxSleep(maxSleep).SetDecayConstant(decayConstant),
+		pacer:       fs.NewPacer(pacer.NewDefault(pacer.MinSleep(minSleep), pacer.MaxSleep(maxSleep), pacer.DecayConstant(decayConstant))),
 		uploadToken: pacer.NewTokenDispenser(fs.Config.Transfers),
 	}
 	f.features = (&fs.Features{