Aftermath of staged parallelism refactoring

[wlandau]

Summary

• Clean up the internals to remove traces of staged parallelism code.
• Refactor predict_runtimes() to match what the schedulers are doing. My intuition says the new predict_runtimes() is much more likely to provide accurate runtime predictions than the previous rendition.
• Deprecate max_useful_jobs(), rate_limiting_times(), and parallel_stages().

GitHub issues addressed

• Ref: A quicker end to staged parallelism #369 (almost there, but not quite done yet)

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

@@          Coverage Diff           @@
##           master   #377    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files          66     64     -2     
  Lines        5518   5359   -159     
======================================
- Hits         5518   5359   -159

Impacted Files	Coverage Δ
R/envir.R	100% <ø> (ø)	⬆️
R/mclapply.R	100% <ø> (ø)	⬆️
R/future_lapply.R	100% <ø> (ø)	⬆️
R/meta.R	100% <ø> (ø)	⬆️
R/deprecate.R	100% <100%> (ø)	⬆️
R/graph.R	100% <100%> (ø)	⬆️
R/predict_runtime.R	100% <100%> (ø)	⬆️
R/migrate.R	100% <100%> (ø)	⬆️
R/timestamp.R	100% <100%> (ø)	⬆️
R/outdated.R	100% <100%> (ø)	⬆️
... and 5 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 663cc92...d1a5c0f. Read the comment docs.