PikiwiDB 支持单 DB

### Which kiwi functionalities are relevant/related to the feature request?

_No response_

### Description

### Which PikiwiDB functionalities are relevant/related to the feature request?

_No response_

### Description

大部分企业在线上不会使用同一个 redis 的多个 DB，pikiwidb 只需要支持单个 DB 即可：

select 0

### Proposed solution

1

### Alternatives considered

1

### Proposed solution

_No response_

### Alternatives considered

_No response_