baseline runs: (6g, gopher.mtv)

http_test.BenchmarkClientServer  5000  412588 ns/op
http_test.BenchmarkClientServer  5000  403346 ns/op
http_test.BenchmarkClientServer  5000  413936 ns/op
http_test.BenchmarkClientServer  5000  410287 ns/op
http_test.BenchmarkClientServer  5000  388037 ns/op
http_test.BenchmarkClientServer  5000  405545 ns/op
http_test.BenchmarkClientServer  5000  405179 ns/op
http_test.BenchmarkClientServer  5000  413827 ns/op
http_test.BenchmarkClientServer  5000  392723 ns/op

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4515155