net/http: optimize http.Dir when files are not modified and add GODEUG to preserve old behavior

[qiulaidongfeng]

DO NOT SUBMIT

Wait Proposal review

Feedback welcome.

TODO: Future supplementary benchmarking

Added a new GODEBUG environment variable.
If GODEBUG=httpdircache=1,
cache *os.File and []byte used by http.Dir.Open
if the open file modification time is not changed.

If GODEBUG=httpdircache=0, the old behavior is retained.

go1.23 Default GODEBUG=httpdircache=1 .

Fixes #64262

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[gopherbot]

This PR (HEAD: 165d6ce) has been imported to Gerrit for code review.

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[gopherbot]

Message from qiulaidongfeng:

Patch Set 2: Run-TryBot+1

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[gopherbot]

Message from Gopher Robot:

Patch Set 2:

(1 comment)

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[gopherbot]

Message from Gopher Robot:

Patch Set 2: TryBot-Result+1

(1 comment)

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