On connection, the new sjRpc work in compatibility mode with the old one. The
label is not defined on frames, and a capabilities special message is sent to the
peer. Old sjRpc ignore this kind on message (but log a warning "malformed
message"), unlike new version which disable this compatibility mode on
receipt.

Now use a callback system to handle event from poller.

When a connection is dropped, all "wait" operations of asynchronous calls
are now released with an error message, exactly like synchronous calls.:

RpcConnection representation doesn't display the socket fd anymore (can cause a bug if the socket is closed).