Respond to server 'start' and 'end' events and add a clock for remaining time

Currently, these events are sent by the server but ignored by the client side
https://github.com/ecvdbdx1617/MultiSnake/blob/dfe307b7f431917b0c4cf69a6f45138c4d7effcd/server/index.js#L47-L55

A game is time-boxed, but the client offers no information about it. Display a clock of remaining time.