[kernel] make more right comment for Commands.
[kernel] fix command default reply, now it like redis have (see 6fa290a)

- Command::REPLY_FALSE
- Command::REPLY_TRUE
- Command::REPLY_NIL
[kernel] use CRLF line separator everywhere in responses.
[kernel] document possible line separators for requests.
[db] response from HGET using protocol specification, instead of just string.