Add a TimerInterface so external packages can mock or spy the timer

[robiningelbrecht]

Currently the Timer class is final, which means it cannot be mocked or extended and that's ok for general usage.
But because of this external packages cannot properly test any classes using the this timer.
Introducing an interface for the timer would fix this problem... or am I missing something?

[sebastianbergmann]

This is outside the scope of this project (as I do not have any use case where I need to isolate Timer from code that uses it for testing).

However, you should not double what you do not own. This means that you should not use Timer directly in your code. Instead, you should define an interface that you own and implement a class that implements your interface using Timer.