fix: if the only stream in a connection times out, prevent re-use

[btasker]

This commit mitigates some raciness which can occur when an established connection fails. Without it, the connection can continue to be drawn from the connection pool, leading to subsequent requests failing.

There may very well be a better way.

Details in golang/go#59690

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Patch Set 1:

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[btasker]

Commit message updated as noted in Gerrit

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Message from Damien Neil:

Patch Set 2:

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Patch Set 2:

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Patch Set 3:

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Patch Set 3:

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