A bit of work on server.js and pre.js

Implemented:

* GNUNET_SERVER_receive_done
* GNUNET_SERVER_client_keep
* GNUNET_SERVER_client_drop
* GNUNET_SERVER_connect_notify
* GNUNET_SERVER_disconnect_notify

Moved some code from pre.js into server.js closer to related code.
Build gnunet-service-cadet and run it