net: Automatically create hidden service, listen on Tor

Starting with Tor version 0.2.7.1 it is possible, through Tor's control socket
API, to create and destroy 'ephemeral' hidden services programmatically.
https://stem.torproject.org/api/control.html#stem.control.Controller.create_ephemeral_hidden_service

This means that if Tor is running (and proper authorization is available),
bitcoin automatically creates a hidden service to listen on, without user
manual configuration. This will positively affect the number of available
.onion nodes.

• When the node is started, connect to Tor through control socket
• Send ADD_ONION command
• First time:
  • Make it create a hidden service key
  • Save the key in the data directory for later usage
• Make it redirect port 8333 to the local port 8333 (or whatever port we're listening on).
• Keep control socket connection open for as long node is running. The hidden service will (by default) automatically go away when the connection is closed.