net/http: new server-only, single-connection keep-alive benchmark

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8046043
net/http: server optimization; reduce GCs, generate ~half the garbage

There was another bufio.Writer not being reused, found with
GOGC=off and -test.memprofile.

benchmark                               old ns/op    new ns/op    delta
BenchmarkServerFakeConnWithKeepAlive        18270        16046  -12.17%

benchmark                              old allocs   new allocs    delta
BenchmarkServerFakeConnWithKeepAlive           38           36   -5.26%

benchmark                               old bytes    new bytes    delta
BenchmarkServerFakeConnWithKeepAlive         4598         2488  -45.89%

Update issue 5100

R=golang-dev, gri
CC=golang-dev
https://codereview.appspot.com/8038047