Introduce timer_is_scheduled() method and replace padding_scheduled_at_us

This came up during legacy/trac#28142 (moved) review: https://github.com/torproject/tor/pull/461#discussion_r231889687

When legacy/trac#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.

changed milestone to %Tor: unspecified in legacy/trac

added component::core tor/tor in Legacy / Trac milestone::Tor: unspecified in Legacy / Trac padding in Legacy / Trac points::1 in Legacy / Trac priority::low in Legacy / Trac refactoring in Legacy / Trac severity::normal in Legacy / Trac sponsor::2 in Legacy / Trac status::new in Legacy / Trac tor-cell in Legacy / Trac tor-relay in Legacy / Trac type::defect in Legacy / Trac wtf-pad in Legacy / Trac labels

Trac:
Keywords: N/A deleted, 041-proposed added

Trac:
Points: N/A to 1

Trac:
Priority: Medium to Low

Trac:
Sponsor: N/A to Sponsor2

Reparent children of legacy/trac#28637 (moved) into legacy/trac#28631 (moved)

Trac:
Parent: legacy/trac#28637 (moved) to legacy/trac#28631 (moved)

Take some wtf-pad stuff out of 041-proposed

Trac:
Keywords: 041-proposed deleted, N/A added

Trac:
Parent: legacy/trac#28631 (moved) to N/A

changed time estimate to 8h

mentioned in issue legacy/trac#29204 (moved)

moved from legacy/trac#28821 (moved)

added Bug label and removed 1 deleted label

added Padding Relay + 1 deleted label and removed 1 deleted label

removed Bug label

removed milestone

closed

added Deferred label

added Technical Debt label and removed 1 deleted label