Current Timer api is hard to use correctly. #945

Related discussion: #944 (comment)
Current timer holds strong reference to TimerHandler, so it's easy to make a strong reference cycle, which leads to memory leak. It needs to be weakened.
Also, TimerLoop::new_timer() requires handler to create TimerApi, but in our use case, it usually holds a object that holds TimerApi, which is impossible on rust without Option<_> type and intermediate mutation.
I think the Timer can be fixed to take care of these situations.