Refactor SlotState to use local interface for DB access

## Issue summary
The type SlotState has a field db of type database.Table

```go
type SlotState struct {
db database.Table
}
```

The interface Table has several methods but the type 'SlotState' use only 'Get' and 'NewBatch' it is better to define a local interface. I explained why in this issue: #3971

## Related issues:
#3975