`ipfs ls` tries to get the contents inside the folder

#### Version information:
go-ipfs version: 0.4.10-
Repo version: 5
System version: amd64/linux
Golang version: go1.8.3

#### Type:
Medium

#### Severity:
Low
#### Description:

Environment:
One folder containing 100,000 files, each of the size 100KB.
Two servers with 1Gb network.
The test folder added in one server and requested by the other one.

I have tested the performance of `ipfs ls`. It seems that `ipfs ls` with the option `--resolve-type=true`, which is the default setting, ipfs will get the contents inside the folder for just listing the folder information (see Fig. 3 and Fig. 4).

I have a quick look of the go-ipfs codes at `go-ipfs/core/commands/ls.go`, which confirms my guess:

```go
// code snippet
output[i] = LsObject{
Hash: paths[i],
Links: make([]LsLink, len(links)),
}

for j, link := range links {
t := unixfspb.Data_DataType(-1)

linkNode, err := link.GetNode(req.Context(), dserv)
if err == merkledag.ErrNotFound && !resolve {
// not an error
linkNode = nil
} else if err != nil {
res.SetError(err, cmds.ErrNormal)
return
}
```

After that, I also tested `ipfs ls --resolve-type=false` and `FUSE + native ls` (see Fig. 1-2 and Fig. 5-6). The results show that with `--resolve-type=false` ipfs will only get the folder info block, and `FUSE + native ls` has the similar behavior with `ipfs ls --resolve-type=true`.

I know that ipfs needs the linked blocks to resolve the data type (in `go-ipfs/unixfs/pb/unixfs.pb.go`):
```go
type Data_DataType int32
const (
Data_Raw Data_DataType = 0
Data_Directory Data_DataType = 1
Data_File Data_DataType = 2
Data_Metadata Data_DataType = 3
Data_Symlink Data_DataType = 4
Data_HAMTShard Data_DataType = 5
)
```

My problems here:
1. In some cases, I only need the folder info. and then get only a few files in the folder. However, ipfs will download the entire folder, which can be extremely slow.
2. Do you have any plan to store the data type information along with the hash links, so that tackling with folders won't be cumbersome.

Thank you.

Related to [#3120](https://github.com/ipfs/go-ipfs/issues/3120)

Fig. 1: **`ipfs ls --resolve-type=false` Sender**

Fig. 2: **`ipfs ls --resolve-type=false` Receiver**

Fig. 3: **`ipfs ls --resolve-type=true` Sender**

Fig. 4: **`ipfs ls --resolve-type=true` Receiver**

Fig. 5: **`ls` with FUSE Sender**

Fig. 6: **`ls` with FUSE Receiver**