net/http: update bundled http2
net/http: update bundled http2

Updates http2 to x/net/http2 git rev 40a0a18 for:

    http2: fix Server race with concurrent Read/Close
    http2: make Server reuse 64k request body buffer between requests
    http2: never Read from Request.Body in Transport to determine ContentLength

Fixes #17480
Updates #17071

Change-Id: If142925764a2e148f95957f559637cfc1785ad21
Reviewed-on: https://go-review.googlesource.com/31737
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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