net/http: more request benchmarks

Add benchmarks for common http benchmarking tools. The intent is to catch optimisations which favor synthetic benchmarks that do not show improvements for real clients like Chrome.

BenchmarkReadRequestChrome        200000             10133 ns/op          60.29 MB/s        3148 B/op         32 allocs/op
BenchmarkReadRequestCurl          500000              4314 ns/op          18.08 MB/s         905 B/op         15 allocs/op
BenchmarkReadRequestApachebench   500000              4363 ns/op          18.79 MB/s         956 B/op         16 allocs/op
BenchmarkReadRequestSiege         500000              6408 ns/op          24.19 MB/s        1397 B/op         22 allocs/op
BenchmarkReadRequestWrk          1000000              2838 ns/op          14.09 MB/s         757 B/op         11 allocs/op

R=golang-dev, bradfitz
CC=golang-dev, haimuiba
https://codereview.appspot.com/7300075