net/http: use pointers to array for copyBufPool

net/http: use pointers to array for copyBufPool

This is inspired by CL 539915, I'm only submitting now that
CL 456435 has been merged.

This divide the number of objects kept alive by the heap by two
and remove the slice header allocation in New and in the put back.

Change-Id: Ibcd5166bac5a37f365a533e09a28f3b79f81ad58
Reviewed-on: https://go-review.googlesource.com/c/go/+/543515
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>