Packets that are sent to multiple clients will now be pre-encoded for
the WebSocket transport (which means simply prepending "4" - which is
the "message" packet type in Engine.IO).

Note: buffers are not pre-encoded, since they are sent without
modification over the WebSocket connection

See also: socketio/engine.io@7706b12

engine.io diff: socketio/engine.io@5.0.0...5.1.0

Syntax:

```js
// server A
io.serverSideEmit("hello", "world");

// server B
io.on("hello", (arg) => {
  console.log(arg); // prints "world"
});
```

With acknowledgements:

```js
// server A
io.serverSideEmit("hello", "world", (err, responses) => {
  console.log(responses); // prints ["hi"]
});

// server B
io.on("hello", (arg, callback) => {
  callback("hi");
});
```

This feature replaces the customHook/customRequest API from the Redis
adapter: socketio/socket.io-redis-adapter#370

A "new_namespace" event will be emitted when a new namespace is created:

```js
io.on("new_namespace", (namespace) => {
  // ...
});
```

This could be used for example for registering the same middleware for
each namespace.

See #3851

Diff: 4.0.2...4.1.0