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 Issue 5100

R=r
CC=adg, dvyukov, gobot, golang-dev, rogpeppe
https://codereview.appspot.com/8819049