Micro optimization

- [x] Prefer async functions over manual promise code
- [x] Run `DebugLogger.enabled()` eagerly, skip `debug()` call during handler invocation (= skip message string creation)
- [x] Try for..in instead of for..of over `Object.keys()`
- [ ] Try to get rid of dynamic creation of `next` callback function
- [ ] Give `const foo = function foo() {` a try
- [ ] Check if all `Response` props should better be non-optional
- [ ] Run with `ndb`, check where time/heap is spent on