Related: #672

In both cases, the error comes from the client as it should not send
multiple concurrent requests, so a HTTP 4xx code is mandated.

Related: #650

Following 24786e7.

Reference: https://expressjs.com/en/guide/error-handling.html

The class used to accumulate the response headers did not expose the
exact same API as its wrapped type, which could lead to the following
error in some rare cases:

> TypeError: Cannot read properties of undefined (reading 'end')
>    at Polling.onDataRequest (build/transports-uws/polling.js:109:53)
>    at Polling.onRequest (build/transports-uws/polling.js:47:18)
>    at callback (build/userver.js:94:56)
>    at uServer.verify (build/server.js:152:9)

Related: socketio/socket.io#4643

In order to prevent issues like:

> error TS2345: Argument of type 'RequestHandler<ParamsDictionary, any, any, ParsedQs, Record<string, any>>' is not assignable to parameter of type 'Middleware'.
>  Types of parameters 'req' and 'req' are incompatible.
>  Type 'IncomingMessage' is missing the following properties from type 'Request<ParamsDictionary, any, any, ParsedQs, Record<string, any>>': get, header, accepts, acceptsCharsets, and 29 more.
>
>  io.engine.use(sessionMiddleware);
                 ~~~~~~~~~~~~~~~~~

Related: socketio/socket.io#4644

We could also have use the RequestHandler type from the
@types/express-serve-static-core package, but that would add 5 new
dependencies.

See also: https://github.com/socketio/engine.io/issues/673

A specially crafted request could lead to the following exception:

> TypeError: Cannot read properties of undefined (reading 'handlesUpgrades')
>    at Server.onWebSocket (build/server.js:515:67)

This bug was introduced in [1], released in version 5.1.0 and included
in version 4.1.0 of the `socket.io` parent package. Older versions are
not impacted.

[1]: 7096e98

Diff: 6.4.1...6.4.2