Keep only one generation of process cache

[contramundum53]

Originally, Kurobako caches all processes that are opened by ExternalProgramProblemFactory. This is for reducing the overhead of creating processes and loading datasets.

However, this behavior can cause a problem when large datasets are loaded in the children processes. Because Kurobako never closes processes even after those processes become unnecessary, all datasets are eventually loaded on memory and that can eat up all memory on the system.

We propose the following changes that mitigate this issue.

• Only keep one generation of cache per thread. In other words, if the command is different from the last one, close the old process.
• Reorder the output of kurobako studies such that the same command is executed consecutively. This is intended to maximize cache efficiency.

[coveralls]

Pull Request Test Coverage Report for Build 2193328853

• 0 of 8 (0.0%) changed or added relevant lines in 2 files are covered.
• 18 unchanged lines in 8 files lost coverage.
• Overall coverage increased (+0.02%) to 9.545%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/study.rs	0	2	0.0%
kurobako_core/src/epi/problem/external_program.rs	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
kurobako_problems/src/nasbench.rs	1	0%
src/main.rs	1	1.61%
src/plot/curve.rs	1	0%
src/problem/rank.rs	1	0%
src/dataset.rs	2	0%
src/runner.rs	2	0%
src/dataset/surrogate.rs	5	0%
src/problem/study.rs	5	0%

Totals
Change from base Build 966921863:	0.02%
Covered Lines:	1101
Relevant Lines:	11535

---

💛 - Coveralls

[sile]

LGTM!

[sile]

Although CI lint checks failed, it seems not relevant to the diff by this PR. So let me fix the lint failures after merging this PR.

[coveralls]

Pull Request Test Coverage Report for Build 2193328853

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 8 (0.0%) changed or added relevant lines in 2 files are covered.
• 18 unchanged lines in 8 files lost coverage.
• Overall coverage increased (+0.02%) to 9.545%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/study.rs	0	2	0.0%
kurobako_core/src/epi/problem/external_program.rs	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
kurobako_problems/src/nasbench.rs	1	0.0%
src/main.rs	1	1.61%
src/plot/curve.rs	1	0.0%
src/problem/rank.rs	1	0.0%
src/dataset.rs	2	0.0%
src/runner.rs	2	0.0%
src/dataset/surrogate.rs	5	0.0%
src/problem/study.rs	5	0.0%

Totals
Change from base Build 966921863:	0.02%
Covered Lines:	1101
Relevant Lines:	11535

---

💛 - Coveralls