This is very primitive and does not have a very good way to find the
total number of elements while loading.

Fix workspaced junctions

Closes #1030

See merge request BuildStream/buildstream!1356

This is the canonical way to fetch sources, and now leverages
the source cache if possible, better to do this rather than
poking around into Element.sources() directly and reimplementing
fetching in the loader.

We already test the behavior without workspaces, lets just augment
these tests to also run with a workspaced junction and check the results.

This guards against regressions of #1030.

Instead of looping over the sources directly, we should be using
the canonical way of checking element consistency.

This requires an additional call to Element._update_state() on the
freshly loaded junction element in order to ensure we calculate consistency.

This fixes issue #1030.

jobs: refactor, use new set_message_unique_id

See merge request BuildStream/buildstream!1337

Ease the burden on subclasses of Job slightly, by providing a new
set_message_unique_id() method. It ensures that created Message
instances will use that id.

This removes the need to override the message() method, so it is no
longer in the 'abstract method' section.

Enable callers to Job's message() method to override the 'unique_id'.

Create artifact proto files atomically

See merge request BuildStream/buildstream!1354