Introduce timer_is_scheduled() method and replace padding_scheduled_at_us
This came up during #28142 (moved) review: https://github.com/torproject/tor/pull/461#discussion_r231889687
When #28142 (moved) gets merged, we can do a small refactoring by removing padding_scheduled_at_us and replacing it with a new timer method that returns true if the timer is scheduled, and false if it's disabled.
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