net/http2: pool transport gzip readers

This is a copy of the CL 510255 with the difference that
it preserves use of fs.ErrClosed when reading after close.

goos: darwin
goarch: arm64
pkg: golang.org/x/net/http2
cpu: Apple M4
              │   HEAD~1    │             HEAD              │
              │   sec/op    │   sec/op     vs base          │
ClientGzip-10   752.8µ ± 1%   750.7µ ± 2%  ~ (p=0.393 n=10)

              │    HEAD~1    │                 HEAD                 │
              │     B/op     │     B/op      vs base                │
ClientGzip-10   75.49Ki ± 0%   33.98Ki ± 3%  -54.99% (p=0.000 n=10)

              │   HEAD~1   │               HEAD                │
              │ allocs/op  │ allocs/op   vs base               │
ClientGzip-10   705.0 ± 0%   698.5 ± 0%  -0.92% (p=0.000 n=10)

Updates golang/go#61353

Change-Id: I0fdc0c0a5947d27dcc615e5bcf4d4620c2c95d9e
GitHub-Last-Rev: 4d04dc93f37ec6cdaf36c6851250806166f4e8c1
GitHub-Pull-Request: golang/net#239
Reviewed-on: https://go-review.googlesource.com/c/net/+/710235
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>