why calculate keyspace ID every time in the request with V2 VPI

[siddontang]

I found that now with V2 API, we will calculate the keyspace every time

func (c *codecV2) GetKeyspaceID() KeyspaceID {
	prefix := append([]byte{}, c.prefix...)
	prefix[0] = 0
	return KeyspaceID(binary.BigEndian.Uint32(prefix))
}

https://github.com/tikv/client-go/blob/master/internal/apicodec/codec_v2.go#L99

Why do we do this? seem that we can cache the value of KeyspaceID in the codecV2 structure directly.

@sunxiaoguang