filer.sync: limit concurrency when fetching file chunks

fix https://github.com/seaweedfs/seaweedfs/issues/3787

weed/replication/sink/filersink/filer_sink.go

@@ -32,6 +32,7 @@ type FilerSink struct {
 	address           string
 	writeChunkByFiler bool
 	isIncremental     bool
+	executor          *util.LimitedConcurrentExecutor
 }
 
 func init() {
@@ -53,6 +54,7 @@ func (fs *FilerSink) IsIncremental() bool {
 func (fs *FilerSink) Initialize(configuration util.Configuration, prefix string) error {
 	fs.isIncremental = configuration.GetBool(prefix + "is_incremental")
 	fs.dataCenter = configuration.GetString(prefix + "dataCenter")
+	fs.executor = util.NewLimitedConcurrentExecutor(32)
 	return fs.DoInitialize(
 		"",
 		configuration.GetString(prefix+"grpcAddress"),