feat: reverse the ping-pong mechanism

The ping packets will now be sent by the server, because the timers set
in the browsers are not reliable enough. We suspect that a lot of
timeout problems came from timers being delayed on the client-side.

Breaking change: v3.x clients will not be able to connect anymore (they
will send a ping packet and timeout while waiting for a pong packet).

Related: https://github.com/socketio/engine.io/issues/312