HELLOHandshake: report server info and switch RESP protocol version (2 or 3).
| Syntax | HELLO [protover] |
| Group | connection |
| Arity | -1 |
| Since | 1.0.0 |
| Flags | readonly |
| Complexity | O(1) |
| Redis compatibility | 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 |