The various background daemon tasks that `arti-client` used to spawn are
now handled inside their respective crates instead, with functions
provided to spawn them that return `TaskHandle`s.

This required introducing a new trait, `NetDirProvider`, which steals
some functionality from the `DirProvider` trait to enable `tor-circmgr`
to depend on it (`tor-circmgr` is a dependency of `tor-dirmgr`, so it
can't depend on `DirProvider` directly).

While we're at it, we also make some of the tasks wait for events from
the `NetDirProvider` instead of sleeping, slightly increasing
efficiency.