x/net/http2: backport critical fixes [1.16 backport]

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A number of fixes in x/net/http2 haven't been backported to 1.16. This is an umbrella issue to figure out which changes should be backported and doing it.

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Change https://golang.org/cl/356980 mentions this issue: [internal-branch.go1.16-vendor] http2: close request body after early RoundTrip failures

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Change https://golang.org/cl/356972 mentions this issue: [internal-branch.go1.16-vendor] http2: use (*tls.Dialer).DialContext in dialTLS

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Change https://golang.org/cl/356979 mentions this issue: [internal-branch.go1.16-vendor] http2: limit client initial MAX_CONCURRENT_STREAMS

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Change https://golang.org/cl/356988 mentions this issue: [internal-branch.go1.16-vendor] http2: refactor request write flow

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Change https://golang.org/cl/356986 mentions this issue: [internal-branch.go1.16-vendor] http2: fix Transport connection pool TOCTOU max concurrent stream bug

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Change https://golang.org/cl/356976 mentions this issue: [internal-branch.go1.16-vendor] http2: close the request body if needed

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Change https://golang.org/cl/357093 mentions this issue: [internal-branch.go1.16-vendor] http2: don't rely on system TCP buffer sizes in TestServer_MaxQueuedControlFrames

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Change https://golang.org/cl/357098 mentions this issue: [internal-branch.go1.16-vendor] http2: don't abort half-closed streams on server connection close

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Change https://golang.org/cl/356975 mentions this issue: [internal-branch.go1.16-vendor] http2: reduce frameScratchBuffer caching aggressiveness

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Change https://golang.org/cl/357089 mentions this issue: [internal-branch.go1.16-vendor] http2: avoid extra GetConn trace call

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Change https://golang.org/cl/356973 mentions this issue: [internal-branch.go1.16-vendor] http2: switch to ASCII equivalents of string functions

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Change https://golang.org/cl/357097 mentions this issue: [internal-branch.go1.16-vendor] http2: on write errors, close ClientConn before returning from RoundTrip