Make hardcoded fetch 100 each worker user configurable

[lxj616]

What would you like to be added:

https://github.com/juicedata/juicefs/blob/main/pkg/sync/cluster.go#L137-L166

Make

				objs = append(objs, obj)
				if len(objs) > 100 {
					break LOOP
				}

User configurable

Why is this needed:

We are using juicefs sync --workers to get 16 very large files from tencent COS to KS3 local jfs

And as we have 8 workers, however 16 is lesser than 100, so all 16 files are using one worker instead of all

Currently our temporary solution is to PAD each big file with 99 empty files, which is, well, working at least

However it would be nicer if I can use juicefs sync --workers on 16 files directly, so

[davies]

We should have a limit on the total size of fetched keys.