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/jottacloud/jottacloud.go

@@ -190,7 +190,7 @@ type Fs struct {
 	endpointURL  string
 	srv          *rest.Client
 	apiSrv       *rest.Client
-	pacer        *pacer.Pacer
+	pacer        *fs.Pacer
 	tokenRenewer *oauthutil.Renew // renew the token on expiry
 }
 
@@ -403,7 +403,7 @@ func NewFs(name, root string, m configmap.Mapper) (fs.Fs, error) {
 		opt:    *opt,
 		srv:    rest.NewClient(oAuthClient).SetRoot(rootURL),
 		apiSrv: rest.NewClient(oAuthClient).SetRoot(apiURL),
-		pacer:  pacer.New().SetMinSleep(minSleep).SetMaxSleep(maxSleep).SetDecayConstant(decayConstant),
+		pacer:  fs.NewPacer(pacer.NewDefault(pacer.MinSleep(minSleep), pacer.MaxSleep(maxSleep), pacer.DecayConstant(decayConstant))),
 	}
 	f.features = (&fs.Features{
 		CaseInsensitive:         true,