Add Clear methods to lru cache

[dakerfp]

No description provided.

[googlebot]

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[dakerfp]

I signed it!

[googlebot]

CLAs look good, thanks!

[bradfitz]

end in a period.

[dakerfp]

Period added.

[dakerfp]

Is there anything else I can do?

[bradfitz]

Squash your two commits down into one.

[dakerfp]

Done.