Add future_lapply_staged parallelism

[wlandau]

Summary

make(parallelism = "future_lapply_staged") is a partial workaround for #448. It will also give us a chance to compare the efficiency of the "future" and "future_lapply" backends against their staged alternative.

Related GitHub issues

• Ref: "future" parallelism takes a really long time to get going when many upstream targets are up to date #448 (not fixed)

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 #450 into master will decrease coverage by 0.01%.
The diff coverage is 97.14%.

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
- Coverage     100%   99.98%   -0.02%     
==========================================
  Files          66       66              
  Lines        5366     5390      +24     
==========================================
+ Hits         5366     5389      +23     
- Misses          0        1       +1

Impacted Files	Coverage Δ
R/future.R	100% <100%> (ø)	⬆️
R/parallel_ui.R	100% <100%> (ø)	⬆️
R/distributed.R	100% <100%> (ø)	⬆️
R/staged.R	99.31% <95%> (-0.69%)	⬇️

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[wlandau]

@kendonB I think this make(parallelism = "future_lapply_staged") might work better for you for now. It even listens to future::plan(workers = n). Merging.

[kendonB]

future_lapply_staged is working beautifully!

[wlandau]

Glad to know! It's a keeper then.