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.

src/main.cpp

@@ -4038,9 +4038,11 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                 CAddress addr = GetLocalAddress(&pfrom->addr);
                 if (addr.IsRoutable())
                 {
+                    LogPrintf("ProcessMessages: advertizing address %s\n", addr.ToString());
                     pfrom->PushAddress(addr);
                 } else if (IsPeerAddrLocalGood(pfrom)) {
                     addr.SetIP(pfrom->addrLocal);
+                    LogPrintf("ProcessMessages: advertizing address %s\n", addr.ToString());
                     pfrom->PushAddress(addr);
                 }
             }