Reduce needless copies

cc @sectioneight @squamos 

This PR eliminates unnecessary uses of Buffer that make node create lots of duplicate copies and spend CPU time calculating things the hard way. I've hit all of the low-hanging-fruit that I can see in the v8 logs, but it appears to have brought share-yo-ride back to a not-pegged CPU level (hovering between 60-80% cpu).

The long-term solution to this is to [add consumer support to this library](https://github.com/sutoiku/node-kafka) so we do the bit banging stuff in C where it belongs, just like the redis client does.