Fix sending multiple metrics per packet through the proxy

Still need to test this more when i'm back in the office and have a lot more metrics to throw at it, but wanted to get this up before i forget, and before we build the update npm package.

Previously we were looping through every metric in the packet, getting the server from the hashring that this metric belongs to, then sending the whole packet with the other metrics as well. Causing multiple statsd instances to receive the same metrics and cause collisions going into Graphite.

@joemiller are most/all of your metrics single metric per packet? if so this wouldn't be causing you any issues, and its why i didn't see any issue in all my testing.

I'll abstract this out a little better and add some tests as well to prevent a regression here again.