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 <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: bradfitz