Use TaskSchedule to sleep in directory bootstrapping

Nick Mathewsonrequested to merge
nickm/arti:dormant_directory into main

The branch adds new APIs to TaskSchedule, and teaches dirmgr to use them in order to make its bootstrapping logic suspendable.

Note that we don't yet return a TaskHandle for the bootstrapping task, because I wasn't so sure of the right API to do so. Any ideas?

Assigning this one to @eta once she's back, since she knows the code both in rtcompat and dirmgr.

Closes #497 (closed)

Edited by Nick Mathewson

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