feat: add the "maxPayload" field in the handshake details

So that clients in HTTP long-polling can decide how many packets they
have to send to stay under the maxHttpBufferSize value.

This is a backward compatible change which should not mandate a new
major revision of the protocol (we stay in v4), as we only add a field
in the JSON-encoded handshake data:

```
0{"sid":"lv_VI97HAXpY6yYWAAAC","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":5000,"maxPayload":1000000}
```

Related: https://github.com/socketio/socket.io-client/issues/1531