Accumulate resources in parallel

[flo-02-mu]

As suggested in #2950 it would be nice to load base resources in parallel. With this PR an additional parameter is introduced:
kustomize build --max_parallel_accumulate n accumulates the resources in parallel go routines.
The default value equals to 1, so that the existing behaviour remains.

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[Shell32-Natsu]

@monopole What do you think about this?

[Shell32-Natsu]

@flo-02-mu Regardless of whether this proposal will be accepted, please add some tests to cover these new behaviors.

[flo-02-mu]

@flo-02-mu Regardless of whether this proposal will be accepted, please add some tests to cover these new behaviors.

In https://github.com/kubernetes-sigs/kustomize/pull/3172/files#diff-840240dda964a03a3ad4437883ccbfbc0b84099f695a083bc7642b7c14df46c6R263 I reused the existing tests but with 16 parallel accumulators. Do you have any specific tests in mind?

[Shell32-Natsu]

Sorry. My fault. But I still want to have more test coverage.

Also waiting for input from @monopole

[flo-02-mu]

@Shell32-Natsu Some existing tests are now duplicated with parallel execution and a dedicated one is added, too.

[monopole]

Before adding this complexity, we all need to see some benchmark tests
https://dave.cheney.net/2013/06/30/how-to-write-benchmarks-in-go
I can think of other ways to improve concurrency, but w/o benchmarks we don't know
that we're solving a problem or just making things more complex.
Would you be willing to add some benchmark test first?

[flo-02-mu]

Would you be willing to add some benchmark test first?

I gave it a try and added a benchmark test: https://github.com/kubernetes-sigs/kustomize/pull/3172/files#diff-840240dda964a03a3ad4437883ccbfbc0b84099f695a083bc7642b7c14df46c6R407
My main intention was to speed up git checkouts, that's why the test uses git endpoints. Benchmark tests are currently not included in the Makefile (and probably it is not a good idea?), so here my local output for the test:

$ go test -bench=.
goos: darwin
goarch: amd64
pkg: sigs.k8s.io/kustomize/api/internal/target
BenchmarkMaxParallelAccumulate1-8              1        43338623097 ns/op
BenchmarkMaxParallelAccumulate2-8              1        31191153303 ns/op
BenchmarkMaxParallelAccumulate3-8              1        25994556832 ns/op
PASS
ok      sigs.k8s.io/kustomize/api/internal/target       101.583s

[Shell32-Natsu]

Please rebase.

It will be good if you can create a distinct PR with a benchmark script so that we can have a base line to measure the performance benefits. It can be a simple script under hack directory.

[flo-02-mu]

@Shell32-Natsu I added a script, but there is not much added value since it just executes go test. An additional test run now does the baseline benchmark without the parallel option set.

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[k8s-ci-robot]

@flo-02-mu: PR needs rebase.

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