store save time in config files

core/java/src/net/i2p/data/DataHelper.java

@@ -514,6 +514,7 @@ public class DataHelper {
             fos = new SecureFileOutputStream(tmpFile);
             out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(fos, "UTF-8")));
             out.println("# NOTE: This I2P config file must use UTF-8 encoding");
+            out.println("# Last saved: " + new Date(System.currentTimeMillis()));
             for (Map.Entry<Object, Object> entry : props.entrySet()) {
                 String name = (String) entry.getKey();
                 String val = (String) entry.getValue();