net: simplify readProtocols via sync.OnceFunc

[1911860538]

In this case, using sync.OnceFunc is a better choice.

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Patch Set 1:

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Patch Set 2:

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Patch Set 2: Commit-Queue+1

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Patch Set 2:

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Patch Set 2: Auto-Submit+1 Code-Review+2 Commit-Queue+1

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Patch Set 2: Code-Review+2

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This PR is being closed because golang.org/cl/666635 has been merged.