Add store API experiment

got you. i propose to change this to 
```
// ListKeysWithPrefix list the keys([]byte) that has the given prefix.
// The listed keys are sorted in (level, bytes) order. Each "/" after prefix gives the key a new order.
// level is the max level of the listed keys. Zero is unlimited.
// list will list keys in the range of [start, end)
ListKeysWithPrefix(horizon uint64, prefix []byte, level, start, end []byte) ([][]byte,error)
```