Restore uber-go/mock.

[jkh52]

Restore uber-go/mock, and make version in README.md consistent with go.mod.

(It looks like #546 was lost, likely inadvertently at #684.)

Includes steps:

# ensure correct tooling
go install go.uber.org/mock/mockgen@v0.5.0
go mod vendor
make mock_gen
go mod tidy

[jkh52]

/assign @cheftako
/cc @liangyuanpeng

[k8s-ci-robot]

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In response to this:

/assign @cheftako
/cc @liangyuanpeng

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[liangyuanpeng]

LGTM

[jkh52]

@cheftako PTAL, thanks!

[jkh52]

/assign @tallclair

[cheftako]

/lgtm
/approve

[tallclair]

/lgtm

[k8s-ci-robot]

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