[MEDIUM] frontend: count the incoming connection earlier

[MEDIUM] frontend: count the incoming connection earlier

The frontend's connection was accounted for once the session was
instanciated. This was problematic because the early ACLs weren't
able to correctly account for the number of concurrent connections.
Now we count the connection once it is assigned to the frontend.
It also brings the nice advantage of being more symmetrical, because
the stream_sock's accept() does not have to account for that anymore,
only the session's accept() does.