slave IO流程之一：mysql登陆过程（mysql_real_connect）

Type    Name    Description
int<1>    header    [00] or [fe] the OK packet header
int<lenenc>    affected_rows    affected rows
int<lenenc>    last_insert_id    last insert-id
if capabilities & CLIENT_PROTOCOL_41 {
int<2>    status_flags    Status Flags
int<2>    warnings    number of warnings
} elseif capabilities & CLIENT_TRANSACTIONS {
int<2>    status_flags    Status Flags
}
if capabilities & CLIENT_SESSION_TRACK {
string<lenenc>    info    human readable status information
if status_flags & SERVER_SESSION_STATE_CHANGED {
string<lenenc>    session_state_changes    session state info
}
} else {
string<EOF>    info    human readable status information
}