Config polling interval does not stop on resetting config

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Split off from #52 where @Messj1 wrote:

BTW: There seems to be a bug 🐛 in the code. The interval get not stopped if someone call ulog.set("log_config", {});

ulog/mods/config/index.js

Lines 43 to 44 in 2aa3419

	set: function(name) {
	if (name === 'log_config') config.update(this)

Maybe a destructor callback would do the trick. At the end, watch.browser.js would look something like that:

module.exports = function(ulog, pollingTimeout) {
  const intervalId = window.setInterval(watchFnc, pollingTimeout);
  return () => {
      window.clearInterval(intervalId);
  }
}

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@Messj1 can you elaborate on this one? I agree that starting a timer and never stopping it is not the best. But I'm not sure where ulog.set('log_config', {}) comes in. In ulog, you only get and set strings. Not objects. Also, the log_config setting is unused in the browser. It allows you to configure the path to the config file on Node JS.