bufio: make Reader buffer transient
Share garbage between different bufio Readers. When a Reader
has zero buffered data, put its buffer into a pool.
This acknowledges that most bufio.Readers eventually get
read to completion, and their buffers are then no longer
needed.
benchmark old ns/op new ns/op delta
BenchmarkReaderEmpty 2993 1058 -64.65%
benchmark old allocs new allocs delta
BenchmarkReaderEmpty 3 2 -33.33%
benchmark old bytes new bytes delta
BenchmarkReaderEmpty 4278 133 -96.89%
Update #5100
R=r
CC=adg, dvyukov, gobot, golang-dev, rogpeppe
https://golang.org/cl/
8819049