Caching options for clustermq_staged parallelism

[wlandau]

Summary

Same as #532, but for parallelism = "clustermq_staged"

Related GitHub issues and pull requests

• Ref: Optionionally allow clustermq workers to cache the results #531

Checklist

• I have read drake's code of conduct, and I agree to follow its rules.
• I have read the guidelines for contributing.
• I have listed any substantial changes in the development news.
• I have added testthat unit tests to tests/testthat to confirm that any new features or functionality work correctly.
• I have tested this pull request locally with devtools::check()
• This pull request is ready for review.
• I think this pull request is ready to merge.

[codecov-io]

Codecov Report

Merging #533 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #533   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          71     71           
  Lines        6421   6440   +19     
=====================================
+ Hits         6421   6440   +19

Impacted Files	Coverage Δ
R/config.R	100% <ø> (ø)	⬆️
R/clustermq.R	100% <100%> (ø)	⬆️
R/staged.R	100% <100%> (ø)	⬆️

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