Adds a `PartialEq` implementation for all models. This sets the stage
for future work in publications and validation, which needs to determine
whether models have changed.

Lays some groundwork for "touch" publications and computing dependency
hashes. These changes need to be paired together, becuase the use of
touch publications is incompatible with our current approach to
dependency change detection.

**Touch operations:**

Are defined as publications where the draft row sets `is_touch: true`
and has a model that is identical to the live model. A touch does not
update the `last_pub_id`, though the agent changes for implementing that
will come in a subsequent commit. The intent is to use a touch in order
to update the `built_spec` in response to changes in dependencies.

**Dependency hash:**

A dependency hash is an opaque value that is computed from the names and
`last_pub_id`s of every spec that a given spec depends on. The concept
of dependencies was introduced along with controllers, but the
implementation of change detection was not fully correct. Using a hash
guarantees (% the infinitesimal chance of hash collision) correctnes of
change detection. This introduces the computation of the hash as part of
validation.

**Other changes:**

Threads through `last_build_id`, and lays some of the groundwork (in
`validation::walk_transition`) for using `last_build_id` instead of
`last_pub_id` for optimistic locking.

Rename `BuiltSpec::is_unchanged` to `is_passthrough`, since the
meaning of `is_unchanged` became unclear with the introduction of touch
operations, which explicitly don't change the model.

Updates the agent to support `is_touch` in publications and the
`dependency_hash` in publications and controllers.

For specs published with `is_touch=true`, the agent will _not_ add new
`publication_specs`, update the `last_pub_id`, or update the `spec`
(the drafted spec must be identical).

The dependency hash column is added, and persisted as part of committing
a publication. Controllers compute the current hash value whenever they
run, and compare it to the persisted value in order to determine whether
any dependencies have changed.

When controllers detect a change in dependencies, their behavior
generally depends on whether any dependencies have been deleted or not.
As long as no dependencies were deleted, then controllers will publish
their spec using `is_touch = true`. If any dependencies have been
deleted, then task controllers will use a normal publication and update
the model to remove the deleted dependencies, which matches the previous
behavior.

Controllers will attempt to collapse multiple subsequent touch
publications into their publication histories in an attempt to limit the
noise of many publications in response to dependency changes.

The `ActivationStatus` has been updated to use `last_build_id` instead
of `last_pub_id` as the high water mark for activations. The previous
behavior was never fully correct, but `last_build_id` must increase
with every modification of the `built_spec`, so this new behavior is
guaranteed not to miss activating any changes.

Going along with the change to `ActivationStatus`, the
`CONTROLLER_VERSION` constant was incremented in order to prevent old
agent versions from clobbering changes to the activation status during
agent rollout.

Updates controllers to set the `detail` for touch publications, to make
debugging easier. Also cleans up a few comments, and skip serializing
`is_touch` in publication histories when it's `false`.

Use consistent field ordering in `tables` and make `is_touch` part of
the return value of `into_parts`.