Add alternative memory management strategies

[wlandau]

Summary

Add a pruning_strategy argument to make() and drake_config() so the user can decide how drake keeps non-import dependencies in memory when it builds a target.

If pruning_strategy is "memory", drake removes all targets from memory (i.e. config$envir) except the direct dependencies of the target is about to build. This is suitable for data so large that the optimal strategy is to minimize memory consumption. If pruning_strategy is "speed", drake loads all the dependencies and keeps in memory everything that will eventually be a dependency of a downstream target. This strategy consumes more memory, but does more to minimize the number of times data is read from storage/disk.

Related GitHub issues

• Ref: Different pruning strategies: memory versus speed #385

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.