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/netbase.h

@@ -215,5 +215,9 @@ std::string NetworkErrorString(int err);
 bool CloseSocket(SOCKET& hSocket);
 /** Disable or enable blocking-mode for a socket */
 bool SetSocketNonBlocking(SOCKET& hSocket, bool fNonBlocking);
+/**
+ * Convert milliseconds to a struct timeval for e.g. select.
+ */
+struct timeval MillisToTimeval(int64_t nTimeout);
 
 #endif // BITCOIN_NETBASE_H