benchmark: rewrite api, update tests

The _bench_timer.js api has been heavily extended and all tests that use
it have been updated. Additions to the new api have been well documented
in the header of the file.
benchmark: add and organize buffer methods

Benchmarks have been created for all Buffer methods. They have also been
placed into benchmark/buffer
benchmark: add and organize net tests

Benchmarks for the net module have been added or moved to the
benchmark/net directory.