Balance policy can not address multi storage medium cluster #704
Description
Describe the bug
Balance policy on cluster with multi storage medium is more complicated.
- When doing balance, a tablet on SSD should be balanced to another SSD medium.
- Some Backends may have SSD, some may not. So we have to calculate the load score in view of storage medium.
- When repairing a tablet, we should choose a Backend even if it does not have a proper storage medium.