[enh] #58: WS2P examples improvements:

- Request blocks around 30.000 as 360.000 does not already exists in Ğ1
- Move dummy credential before getpass in order to be overwriten
- Add ability to pass member credentials to listen_ws2p
- Add getpass import
- Close WS2P clients in three examples in case of a raised exception
- Use sys.exit() because pylint complains