sync: add a buffer for checks, uploads and renames #379

--max-backlog controls the queue length.

Add statistics for the check/upload/rename queues.

This means that checking can complete before the uploads which will
give rclone the ability to show exactly what is outstanding.

fs/config/configflags/configflags.go

@@ -83,6 +83,7 @@ func AddFlags(flagSet *pflag.FlagSet) {
 	flags.FVarP(flagSet, &fs.Config.StreamingUploadCutoff, "streaming-upload-cutoff", "", "Cutoff for switching to chunked upload if file size is unknown. Upload starts after reaching cutoff or when file ends.")
 	flags.FVarP(flagSet, &fs.Config.Dump, "dump", "", "List of items to dump from: "+fs.DumpFlagsList)
 	flags.FVarP(flagSet, &fs.Config.MaxTransfer, "max-transfer", "", "Maximum size of data to transfer.")
+	flags.IntVarP(flagSet, &fs.Config.MaxBacklog, "max-backlog", "", fs.Config.MaxBacklog, "Maximum number of objects in sync or check backlog.")
 }
 
 // SetFlags converts any flags into config which weren't straight foward