BR should filter out TiFlash nodes

[kennytm]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

Backup from a cluster where some TiFlash nodes are running

2. What did you expect to see?

Success.

3. What did you see instead?

Backup failed with "Unimplemented" error in SendBackup(), on this line:

br/pkg/backup/client.go

Line 684 in 328ad69

resp, err := bcli.Recv()

Since TiFlash cannot participate in BR anyway, we should just filter these nodes out.

4. What version of BR and TiDB/TiKV/PD are you using?

3.1.0-beta.1

[jebter]

This bug should block 3.1.0 GA, please fix it.

[kennytm]

@jebter do you know how to distinguish between a TiKV node and TiFlash node? PD's /pd/api/v1/stores API doesn't contain such information.

[jebter]

The information of the TiFlash node contains a label:
"labels": [
           {
             "key": "engine",
             "value": "tiflash"
           }
         ]
cc @nolouch