Remove SocketConnected and SocketDisconnected bus events

I've removed SocketConnected and SocketDisconnected bus events from Zebus since it's not longer useful. 
I've removed the socket events on ITransport interface too but I've choose to leave an internal [SocketDisconnected](https://github.com/Abc-Arbitrage/Zebus/blob/remove-socket-events/src/Abc.Zebus/Transport/ZmqTransport.cs#L71) event on ZmqTransport for test purpose. 

The related tests are :
* [should_terminate_zmq_connection_of_a_forgotten_peer_after_some_time](https://github.com/Abc-Arbitrage/Zebus/blob/remove-socket-events/src/Abc.Zebus.Tests/Transport/ZmqTransportTests.cs#L224)
* [should_terminate_zmq_connection_of_a_started_peer_with_no_delay](https://github.com/Abc-Arbitrage/Zebus/blob/remove-socket-events/src/Abc.Zebus.Tests/Transport/ZmqTransportTests.cs#L257)
* [should_disconnect_peer_socket_of_a_stopped_peer_after_some_time](https://github.com/Abc-Arbitrage/Zebus/blob/remove-socket-events/src/Abc.Zebus.Tests/Transport/ZmqTransportTests.cs#L505)