Add timed activity (or similar) for waiting for buildbox-casd

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1301)
In GitLab by [[Gitlab user @chiaratolentino]](https://gitlab.com/chiaratolentino) on May 5, 2020, 14:56

## Background

[//]: # (Provide a background or the root/source that justifies this task or action.)

As discussed in !1889, with the longer ``buildbox-casd`` timeout period, we should have some way of telling users what's going on (e.g. a timed activity or periodic status telling the user that we're still waiting for ``buildbox-casd`` to start) without generating too much noise.

See:
- https://gitlab.com/BuildStream/buildstream/-/merge_requests/1889#note_333881011
- https://gitlab.com/BuildStream/buildstream/-/merge_requests/1889#note_336027502

## Acceptance Criteria

[//]: # (Acceptance criteria should follow the S.M.A.R.T. principle https://en.wikipedia.org/wiki/SMART_criteria )
Users must get some kind of feedback/message to tell them when we're waiting on buildbox-casd to start

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[//]: # (To review information about possible relevant labels for this issue please view the list of labels: https://gitlab.com/BuildStream/buildstream/labels)

Start by reading the discussion in merge request !1889, especially the linked notes, to understand the longer buildbox-casd timeout and the desired feedback behavior. Trace the existing wait path for buildbox-casd and identify how timed activity or periodic status is reported; done means users receive non-noisy feedback while waiting for it to start.

Written by the indexing model from the issue text.