Non blocking errors was not handled in the writer callback.

When given a non RPC handler callback, it didn't work.

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.

These methods must be available only through the Rpc protocol. To access
to the default RPC (aka rpc0), use the "connection.rpc" property:

    conn.rpc.set_handler(MyHandler())

This behavior allow to handle properly the case where an error
(such as a Broken Pipe error) is raised when reading on the socket.
When using SimpleServer class, the socket will be removed from the
connected client list. When using as client, the application is in charge
of re-connection exactly like a desired shutdown. A DEBUG logging message
is emitted by the library when this event is triggered.

RpcError are now reserved to rpc protocol, the RpcConnection uses the new
RpcConnectionError class.

NoFreeLabelError and FallbackModeEnabledError are now extending this new
class.