Parses the response packet to recognise Ok and Error packets.
Returns an _OkPacket if the packet was an Ok packet, throws
a MySqlException if it was an Error packet, or returns null
if the packet has not been handled by this method.
Parses a Buffer containing the response to the command.
Returns a _HandlerResponse.
The default implementation returns a finished _HandlerResponse with
a result which is obtained by calling checkResponse