net/http: reuse bufio.Reader and bufio.Writer between conns
net/http: reuse bufio.Reader and bufio.Writer between conns

Saves over 8KB of allocations per new connection.

benchmark                             old ns/op    new ns/op    delta
BenchmarkServerFakeConnNoKeepAlive        28777        24927  -13.38%

benchmark                            old allocs   new allocs    delta
BenchmarkServerFakeConnNoKeepAlive           52           46  -11.54%

benchmark                             old bytes    new bytes    delta
BenchmarkServerFakeConnNoKeepAlive        13716         5286  -61.46%

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7799047

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