Backend performance

[wrossmann]

We have a large amount of perfdata we're pushing through to Carbon and I've found that Graphios is capping out at about 7k metrics per second. In order to get through a backlog of perfdata I moved the older data to a separate spool folder and started a second Graphios process which is now happily throwing another 7k metrics per second into my Graphite infra.

For this I would like to raise 2 issues:

1. In the Carbon backend the socket is set up and torn down each time send() is called. I've observed that this causes a significant amount of overhead while Graphios is processing metrics.
2. The current single-threaded nature of Graphios is holding it back. I believe that building in the ability to run two or more worker threads within a single instance of Graphios would greatly improve performance.