fs: Change default WriteStream config, increase perf

This increases fs.WriteStream throughput dramatically by removing the
"higher default water marks" for fs.WriteStream.

Also includes a benchmark.  Current performance is significantly higher
than v0.8 for strings at all tested levels except size=1.  Buffer
performance is still lackluster.

Further improvement in the stream.Writable base class is required, but
this is a start.

Related to #4706

Review, please: @bnoordhuis