hdfs: fix retry "replication in progress" errors when uploading

Before this change uploaded files could return the error "replication
in progress".

This error is harmless though and means the Close should be retried
which is what this patch does.

backend/hdfs/fs.go

@@ -21,6 +21,7 @@ import (
 	"github.com/rclone/rclone/fs/config/configmap"
 	"github.com/rclone/rclone/fs/config/configstruct"
 	"github.com/rclone/rclone/fs/hash"
+	"github.com/rclone/rclone/lib/pacer"
 )
 
 // Fs represents a HDFS server
@@ -31,8 +32,15 @@ type Fs struct {
 	opt      Options        // options for this backend
 	ci       *fs.ConfigInfo // global config
 	client   *hdfs.Client
+	pacer    *fs.Pacer // pacer for API calls
 }
 
+const (
+	minSleep      = 20 * time.Millisecond
+	maxSleep      = 10 * time.Second
+	decayConstant = 2 // bigger for slower decay, exponential
+)
+
 // copy-paste from https://github.com/colinmarc/hdfs/blob/master/cmd/hdfs/kerberos.go
 func getKerberosClient() (*krb.Client, error) {
 	configPath := os.Getenv("KRB5_CONFIG")
@@ -114,6 +122,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 		opt:    *opt,
 		ci:     fs.GetConfig(ctx),
 		client: client,
+		pacer:  fs.NewPacer(ctx, pacer.NewDefault(pacer.MinSleep(minSleep), pacer.MaxSleep(maxSleep), pacer.DecayConstant(decayConstant))),
 	}
 
 	f.features = (&fs.Features{