HELLOHandshake: report server info and switch RESP protocol version (2 or 3).
| 语法 | HELLO [protover] |
| 分组 | connection |
| 参数个数 | -1 |
| 起始版本 | 1.0.0 |
| 标志 | readonly |
| 复杂度 | O(1) |
| Redis 兼容性 | differs: only HELLO [2|3] is honoured — the AUTH and SETNAME tails are parsed off and ignored, because kevy has no AUTH and this form does not set the connection name |