bufio: reuse Writer buffers after Flush

bufio: reuse Writer buffers after Flush

A bufio.Writer.Flush marks the usual end of a Writer's
life. Recycle its internal buffer on those explicit flushes,
but not on normal, as-needed internal flushes.

benchmark               old ns/op    new ns/op    delta
BenchmarkWriterEmpty         1959          727  -62.89%

benchmark              old allocs   new allocs    delta
BenchmarkWriterEmpty            2            1  -50.00%

benchmark               old bytes    new bytes    delta
BenchmarkWriterEmpty         4215           83  -98.03%

R=gri, iant
CC=gobot, golang-dev, voidlogic7
https://golang.org/cl/9459044