bytes: don't compact Buffer so aggressively

benchmark                           old ns/op    new ns/op    delta
BenchmarkBufferNotEmptyWriteRead       848416       819983   -3.35%

Update Issue 5154

R=golang-dev, gri, robryk
CC=golang-dev
https://codereview.appspot.com/8173043
runtime: additional map benchmarks for repeated lookups

For the future.

Update Issue 5147

R=khr, r
CC=golang-dev
https://codereview.appspot.com/8165044