convert waiter from file monitor to timer/ticker combination

[gabemontero]

We could maximize code reuse, via golang constructs of Timer and Ticker, and convert cmd/waiter/waiter.go from #934 and its loop on the PID file maintenance.

Instead, you can have a single select with cases for both the Timer and Ticker.

1. the Timer would capture the current timeout capability. If it pops, it stops/cancels the Ticker, and the waiter gracefully exits having timed out / quit waiting for the condition to complete.
2. the waiter's Done method that is called when the "condition" is achieved calls the Ticker cancel/stop, and that event leads to a stop/cancel of the Timer, and the waiter gracefully exits, have successfully waited for its desired condition.

See the thread starting at #934 (comment) for more details

[gabemontero]

OK after having looked at the merged code in my editor (easier for me to visualize than github PR code diff presentation :-) ) and better seeing/realizing the multi process nature of waiter start then waiter done, I now realize this change is really untenable. Certainly if we constructed a multiple process communication to facilitate the timer/ticker, it would be no simpler than the current file based approach.

closing

@otaviof FYI