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 #5100

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/8038047