fs: Change default WriteStream config, increase perf

[isaacs]

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

[bnoordhuis]

LGTM. It doesn't seem to be unequivocally faster (i.e. faster in all cases) but I'm testing it on a MBA so take with copious amounts of salt.

[isaacs]

Landed on master. Further benchmark analysis shows that this is just the tip of the iceberg, but it's a start.

[trevnorris]

this change is causing test-http-get-pipeline-problem to fail sporadically with Abort (core dumped) on my system. tested on linux with clang 3.2 and gcc 4.7 for x64 and ia32 builds. all have the same results.

@isaacs revert until fix is found?