net: fix deadlock in lookupProtocol on Windows

[spikecurtis]

If the context expires before acquireThread(ctx) succeeds, then the goroutine can block like:

net.lookupProtocol.func1()
src/net/lookup_windows.go:58 +0x105
created by net.lookupProtocol in goroutine 2834
src/net/lookup_windows.go:56 +0xda

We saw this in our UTs with a leak detector, confirmed by inspection of the source code.

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Patch Set 1:

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Patch Set 2: Code-Review+1

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Patch Set 2: Commit-Queue+1

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Dry run: CV is trying the patch.

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Patch Set 2:

(3 comments)

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Patch Set 2: -Commit-Queue

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Patch Set 3:

(2 comments)

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Patch Set 3: Code-Review+2

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Patch Set 3: Code-Review+1

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