Allow evict-slow-store-scheduler to schedule when there are more than one slow stores

## Feature Request

### Describe your feature request related problem

Today, the evict-slow-store-scheduler schedules when there is exactly one slow store in the cluster, when there are more than one store with slowScore 100, the scheduler skips the scheduling, and it happened in our chaos testing. We believe that it would be very helpful to still schedule in the multiple slow store scenario.

### Describe the feature you'd like

One of the two options to consider

- [ ] Once a slow store with `slowScore >= slowStoreEvictThreshold` is identified, proceed to start evicting leaders from that store until the store has been removed or recovered.
- [ ] Add a configurable max-slow-store-to-evict, e.g. 2 slow stores can be evicted in the scheduler simultaneously, with the higher slowScore being the priority in each run.

### Describe alternatives you've considered

### Teachability, Documentation, Adoption, Migration Strategy