Fix timer callback triggering after timer is cleared

[chandlerprall]

Fixes elastic/kibana#19285

We could get fancy and fix this specific situation through more coordination, or we can be defensive and just not trigger the callback if it's been removed. I opted for the second because of its simplicity and it addresses any related bugs that may occur in the future.

This also fixes a second bug where the toast's countdown is not restarted for the correct amount of time after the second hover.

what causes the error

When a toast managed by `EuiGlobalToastList` reaches the end of its timer, the timer self-clears and triggers the callback, beginning the process of fading out the element. The toast fade-and-remove can be laid out as

• timer's timeout happens
  • timer.callback()
    • toast list tracks this toast as dismissed (it starts fading out)
    • toast list sets timeout X
  • timer.clear()
    • removes timer callback
• timeout X happens
  • toast dismissal callback happens, which removes toast from the list
  • toast's timer is cleared, to be safe (allowing dismissals outside of this flow)

The bug as described in the kibana issue can happen by a 3rd event triggering between the timer's timeout and timeout X: EuiGlobalToastList's onMouseLeave handler cycles through the known timers and calls resume, re-creating their setTimeouts. The list of known timers at this point includes elements that are fading out, which have already completed their timers, and their callbacks have been removed.

[rhoboat]

LGTM!

[rhoboat]

This looks good to me, but I'm just wondering: these methods all look like they could benefit from some awaits or promise handling. Wherever Timer is used, there might be a need to chain events making sure the previous function finished executing. I'm just a little surprised looking through this class, that it doesn't await this.callback(). I guess nothing that calls finish needs to be chained with something else?