Balance policy can not address multi storage medium cluster

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