net/http/cookiejar: implement concurrent-safe Clear method

[AidanWelch]

Currently clearing all stored cookies puts a needless burden on package consumers, especially when used within http/Client where the calls to the cookiejar are not directly done by the consumer. This change just directly implements a safe clear of all entries.

Fixes #70258

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 1: Hold+1

(1 comment)

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