feat: do not reuse the Engine.IO id

In previous versions, the Socket#id attribute was equal (or derived,
for a non-default namespace) to the underlying Engine.IO id, which is
used as a mean to authenticate the user throughout the Engine.IO
session and thus is sensitive information that should be kept secret.

The problem with reusing the Engine.IO id is that users could be
tempted to transmit this id to other clients, in order to implement
private messaging for example.

So we'll now generate a new random id for each new socket.

Please note that this id will now be different from the one found in
the query parameters of the HTTP requests.