In order to ease the migration to Socket.IO v3, the Socket.IO server
can now communicate with v2 clients.

```js
const io = require("socket.io")({
  allowEIO3: true
});
```

This feature is disabled by default.

Reference: https://socket.io/docs/#Other-client-implementations

When handling compression at the proxy server level, the client receives a weak ETag.
Weak ETags are prefixed with `W/`, e.g. `W/"2.2.0"`.
Upon cache validation we should take care of these too.

Reference: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag

Diff: socketio/socket.io-adapter@2.0.3...2.1.0

Includes:

- add room events ([155fa63](socketio/socket.io-adapter@155fa63))
- make rooms and sids public ([313c5a9](socketio/socket.io-adapter@313c5a9))

Diff: 3.0.5...3.1.0