capacity: add benchmark for CSIStorageCapacity update

[pohly]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:

It may be possible to optimize updating capacity by using patching. Before
trying that we need a benchmark.

Which issue(s) this PR fixes:

Fixes #467

Special notes for your reviewer:

This benchmark acts as a client with basically unlimited local rate
limits. When run long enough, the results are stable:

$ go test -run=xxx -bench=. -count=5 -benchtime=1m . | tee /tmp/log
...
$ $GOPATH/bin/benchstat /tmp/log
name               time/op
CapacityUpdate-36  3.93ms ± 1%

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: pohly
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[k8s-ci-robot]

@pohly: PR needs rebase.

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[pohly]

There doesn't seem to be enough interest in having these benchmarks included in the repo.

/close

[k8s-ci-robot]

@pohly: Closed this PR.

In response to this:

There doesn't seem to be enough interest in having these benchmarks included in the repo.

/close

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