Micro optimization

[andywer]

• Prefer async functions over manual promise code
• Run DebugLogger.enabled() eagerly, skip debug() call during handler invocation (= skip message string creation)
• 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