Use TaskSchedule to sleep in directory bootstrapping

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)