Fixed bug with server's client connection management

A server need to keep a list of connected clients to trigger their
shutdown when a global server shutdown is requested. To avoid memory
leaks with the client list, the list is composed of weakref. Problem:
connected RpcConnection can be collected because the application is not
required to keep a ref on each connection.

The problem was fixed by removing weakrefs. The list is cleaned of its
offline clients each time a new client is connected.