Store server tunnel keys / read tunnel keys from a specified directory when creating a tunnel with tunnelcontrol

Use .i2p-zero in the user's home directory as the i2p config dir.

org.getmonero.i2p.zero/src/org/getmonero/i2p/zero/TunnelControl.java

@@ -36,8 +36,9 @@ public class TunnelControl implements Runnable {
     this.router = router;
     tunnelControlTempDir.delete();
     tunnelControlTempDir.mkdir();
+
     this.tunnelControlTempDir = tunnelControlTempDir;
-    this.tunnelControlTempDir.deleteOnExit();
+    Runtime.getRuntime().addShutdownHook(new Thread(()->this.tunnelControlTempDir.delete()));
   }
 
   public interface Tunnel {