tor: Add option to encrypt Tor private key

This commit lays the groundwork for enabling the option of encrypting a Tor private key on disk, and removes the onion type parameters from the OnionStore interface methods, since they are unused.

tor/onionaddr.go

@@ -45,6 +45,9 @@ type OnionAddr struct {
 
 	// Port is the port of the onion address.
 	Port int
+
+	// PrivateKey is the onion address' private key.
+	PrivateKey string
 }
 
 // A compile-time check to ensure that OnionAddr implements the net.Addr