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.

Don't use SystemTime::now()

Closes #306

See merge request tpo/core/arti!365

(Doing this a few days ahead of time to give updated code time to
get testing)

implement IsolationHelper for StreamIsolation

See merge request tpo/core/arti!434

Fixed typo on arti-client README.md

See merge request tpo/core/arti!432

arti-client, dirmgr: Initial DirFilter code

See merge request tpo/core/arti!431

Simplify DirFilter API

See merge request nickm/arti!1

This means you can write a DirFilter and only write code to mess with
the bits you want to.

Also, it is less code here.

There are two reasons why the DynFilter newtype might be needed:

1. To impl Default.  But we don't need it to impl Default since we can
have an accessor which does the defaulting.

2. To hide the API.  But this is usrely an unstable API.

Just writing Arc<dyn> gets rid of a lot of unnecessary boilerplate and
conversion code.

Abolish the handwritten Debug impl for DynFilter, which is no longer
needed.

Implement a periodic task scheduler, and a basic dormant mode

See merge request tpo/core/arti!429

but don't use it in a dyn Isolation context

This will make it possible to implement a directory-munging
mechanism in arti-testing for #397.

This code sits behind a feature flag, and can be used to modify
directories before storing them.  This is part of the implementation
for #397.

Addressing review comments: added some unit tests for the new scheduler
type, and made FireIn use an Instant instead (making it FireAt).

This is a revised version of !397; it implements a scheduling system for
periodic tasks that can be externally controlled, and then uses the
external control aspect to implement a basic dormant mode (#90).

More technically, the scheduling system consists of a `Stream` that
periodic tasks are expected to embed in a `while` loop or similar, a
way for tasks themselves to choose how long to wait until the stream
next yields a result, and a handle to control this outside of the task.

tor-proto: add the ability to learn clock skew from NETINFO cells

See merge request tpo/core/arti!410

We now check the handshake certificates unconditionally, and only
report them as _expired_ as a last resort.

(Rationale: if somebody is presenting the wrong identity from a year
ago, it is more interesting that they are presenting the wrong ID
than it is that they are doing so with an expired cert.

We also now report a different error if the certificate is expired,
but its expiration is within the range of reported clock skew.

(Rationale: it's helpful to distinguish this case, so that we can
blame the failure on possible clock skew rather than definitely
attributing it to a misbehaving relay.)

Part of #405.

NETINFO cells, which are sent in every handshake, may contain
timestamps.  This patch adds an accessor for the timestamp in the
Netinfo messages, and teaches the tor-proto code how to compute the
minimum clock skew in the code.

The computation isn't terribly precise, but it doesn't need to be:
Tor should work fine if your clock is accurate to within a few
hours.

This patch also notes a Y2038 problem in the protocol: see
torspec#80.

Part of #405.

Move SystemConfig to arti::cfg

Closes #418 and #314

See merge request tpo/core/arti!427

Expand documentation for isolation traits

See merge request tpo/core/arti!420

Change rules for retrying guards (and marking them broken)

See merge request tpo/core/arti!426

Adjust a comment

See merge request tpo/core/arti!428

Noticed this while reviewing !426

Pattern bindings after `@` weren't stabilized then :/