Support for Decouping of Compute and Storage of TiFlash

## Feature Request

### Describe your feature request related problem

### Describe the feature you'd like

The feature requests are from the **Decoupling of Compute and Storage for TiFlash**.
- [ ] Add new label for TiFlash ReadNode: Now the label of **TiFlash** is , we need to to add a new lable for tiflash nodes which only used for MPP computation(aka ReadNode). And the region data will be stored in WriteNode.
- [ ] Add node cache information(like region cache), then TiDB use node information when dispatch MPPTask.
### Describe alternatives you've considered

### Teachability, Documentation, Adoption, Migration Strategy