Add support for resetting a timer

[RussellLuo]

Motivation

Implement something like Timer.Reset, to reuse an existing timer and reset its duration.

Solutions

There are three candidate solutions:

1. Add Timer.Reset

func (t *Timer) Reset(d time.Duration) {
    ...
}

Pros:

• The API is similar to Timer.Reset, which is simple and intuitive.

Cons:

• Since Reset needs to add the timer back into the timing wheel, the timer must know which timing wheel it belongs to.
• To achieve this goal, we need to add a new field (e.g. tw *timingwheel.TimingWheel) into the Timer struct. And this is a waste of memory since the total amount of timers is often large.

2. Add TimingWheel.ResetTimer

func (tw *TimingWheel) ResetTimer(t *Timer, d time.Duration) {
    ...
}

Pros:

• There is no waste of memory.

Cons:

• The API is simple but not as intuitive as Timer.Reset.

3. Modify TimingWheel.AfterFunc

Add one more argument named t to provide an existing timer. If t is not nil, we reuse it. Otherwise, we create a new timer.

func (tw *TimingWheel) AfterFunc(d time.Duration, f func(), t *Timer) *Timer {
    ...
}

Pros:

• There is no waste of memory.
• There is only one API AfterFunc, no need to add a new one.

Cons:

• AfterFunc becomes more complex.